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.