Why was Snape so concerned with exams if he wanted to join the Death Eaters?

Question

Snape is shown to be putting a great deal of effort into his exam in Order of the Phoenix. He's writing in answers up until the last minute. After the exam Snape is still reviewing his notes, oblivious to James and Sirius. But in Deathly Hallows we see Lily confronting him about his true goal- to join Voldemort and become a Death Eater:

You and your precious little Death Eater friends – you see, you don’t even deny it! You don’t even deny that’s what you’re all aiming to be! You can’t wait to join You-Know-Who, can you?’ He opened his mouth, but closed it without speaking.

This implies that Lily had picked up on this fact, even though Snape had not verbalized it. However, I doubt Snape obsessed over his exams to cover up his actual goal for Lily’s (or anyone's) sake, so why did he bother worrying so much about his OWLs? I understand he had an appetite for knowledge, but it's not as if Voldemort was asking new recruits what sort of grades they received.

Answer 1

Snape personally valued knowledge.

Though Snape may have planned to join the Death Eaters, that does not mean he would therefore not care about his grades because Voldemort and the Death Eaters would not check them. Snape probably still personally cared about his studies, and furthermore was eager to apply himself in class and gain knowledge. From a young age, Snape seems to have valued intelligence. He snidely comments to James that Gryffindor is for those who would rather be brawny than brainy, implying he valued intelligence more.

“No,’ said Snape, though his slight sneer said otherwise. ‘If you’d rather be brawny than brainy –”
_{^{- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)}}

He studied presumably quite a bit before attending Hogwarts, and particularly studied the Dark Arts. This shows his innate thirst for knowledge, and additionally would indeed be considered useful for a Death Eater. Though the Death Eaters would not care about grades specifically, intelligent wizards would be more highly valued over unintelligent ones.

“Snape's always been fascinated by the Dark Arts, he was famous for it at school. Slimy, oily, greasy-haired kid, he was," Sirius added, and Harry and Ron grinned at each other. "Snape knew more curses when he arrived at school than half the kids in seventh year and he was part of a gang of Slytherins who nearly all turned out to be Death Eaters.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)}}

Snape was not the only Hogwarts student with a particular interest in the Dark Arts and plans for after attending Hogwarts that would not require good grades - Voldemort also excelled in his examinations despite good grades not being necessary to be a Dark Lord.

“Before we see what Hokey witnessed, I must quickly recount how Lord Voldemort left Hogwarts.

‘He reached the seventh year of his schooling with, as you might have expected, top grades in every examination he had taken.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 20 (Lord Voldemort’s Request)}}

Snape retained this valuing of intelligence in his adult life. His home has walls completely covered with books, and as this is not at Hogwarts, this is presumably his personal collection.

“The walls were completely covered in books, most of them bound in old black or brown leather; a threadbare sofa, an old armchair and a rickety table stood grouped together in a pool of dim light cast by a candle-filled lamp hung from the ceiling.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 2 (Spinner’s End)}}

From the way he speaks to his class, it seems clear that Snape has little time for those who choose to remain willfully ignorant by not paying attention in class.

“You are here to learn the subtle science and exact art of potion-making,’ he began. He spoke in barely more than a whisper, but they caught every word – like Professor McGonagall, Snape had the gift of keeping a class silent without effort. ‘As there is little foolish wand-waving here, many of you will hardly believe this is magic. I don’t expect you will really understand the beauty of the softly simmering cauldron with its shimmering fumes, the delicate power of liquids that creep through human veins, bewitching the mind, ensnaring the senses … I can teach you how to bottle fame, brew glory, even stopper death – if you aren’t as big a bunch of dunderheads as I usually have to teach.”
^{- Harry Potter and the Philosopher's Stone, Chapter 8 (The Potions Master)}

Therefore, Snape likely still cared about his studies on a personal level, regardless of whether they were required for him to be allowed to join the Death Eaters.

Answer 2

"Death Eater" is not a job.

We see many Death Eaters who have other jobs - especially in the Ministry of Magic, where they worked as double-agents for Voldemort - some of which attended Hogwarts at approximately the same time as Snape.

Having good grades, and then being able to get a useful position in society (such as working for the Ministry or Hogwarts) would raise Snape's utility as a Death Eater, helping him to rise in the ranks.

Answer 3

The reason Lily had that conversation with Snape was that he called her a mudblood. I think that’s a critical point because we know Snape is half-blood.

I don’t have quotes on this, but based on real life analogies, I can guess that it wouldn’t have been easy to fit in with Slytherin if almost everyone else were pure blood fanatics. He wasn’t from a wealthy pureblood family and he definitely wasn’t brawny. The only thing he had really was his knowledge.

Voldemort cared about high achievers. But I assume he didn’t run recruitment operations to test skills. Instead, people like Malfoy helped pick out potential recruits. This is hinted at in Snape’s memory of first being sorted into Slytherin and being welcomed by Malfoy. If their initial conversation follows the same track as Harry’s first dinner in Hogwarts, then it would’ve been apparent before his first bite that Snape was an outsider in Slytherin.

The only way an outsider who is brainy can get their attention is by validation of his knowledge through scores. He knew that Voldemort was a top student and also that rumors of his parentage died down after his fifth year.

”I don't know that politics would suit me sir" he said when the laughter died away. "I don't have the right kind of background, for one thing.” A couple of the boys around him smirked at each other. Harry was sure they were enjoying a private joke, undoubtedly about what they knew, or suspected, regarding their gang leader's famous ancestor. "Nonsense," said Slughorn briskly, " couldn't be plainer you come from decent wizarding stock, ability like yours."

-- Harry Potter and the Half Blood Prince. Chapter Twenty Three - Horcruxes.

Since knowledge of Chamber of Secrets wasn’t widely known, any smart person would assume that the rumors died down because of his top OWLs.