Intermediate results are informative.
While a high-quality final product is the ultimate goal of any potion recipe, students can gain longstanding skills by learning about methods and indications throughout the creation process.
For example, the procedure to create the Draught of Peace, although brief enough to complete during a single lesson, demonstrates that intermediate results provide useful quality-control information to the potion-maker:
'A light silver vapour should now be rising from your potion,' called Snape, with ten minutes left to go.
Harry, who was sweating profusely, looked desperately around the dungeon. His own cauldron was issuing copious amounts of dark grey steam; Ron's was spitting green sparks. Seamus was feverishly prodding the flames at the base of his cauldron with the tip of his wand, as they seemed to be going out. The surface of Hermione's potion, however, was a shimmering mist of silver vapour, and as Snape swept by he looked down his hooked nose at it without comment, which meant he could find nothing to criticise. At Harry's cauldron, however, Snape stopped, and looked down at it with a horrible smirk on his face.
'Did you do everything on the third line, Potter?'
'No,' said Harry very quietly.
'I beg your pardon?'
'No,' said Harry, more loudly. 'I forgot the hellebore.'
'I know you did, Potter, which means that this mess is utterly worthless.'
— Harry Potter and the Order of the Phoenix, Chapter 12: "Professor Umbridge"
The Draught of Living Death offers similar feedback:
Harry stirred counter-clockwise, held his breath, and stirred once clockwise. The effect was immediate. The potion turned palest pink.
'How are you doing that?' demanded Hermione, who was red-faced and whose hair was growing bushier and bushier in the fumes from her cauldron; her potion was still resolutely purple.
'Add a clockwise stir –'
'No, no, the book says counter-clockwise!' she snapped.
Slughorn moved slowly between the tables, peering into cauldrons. He made no comment, but occasionally gave the potions a stir, or a sniff. At last he reached the table where Harry, Ron, Hermione and Ernie were sitting. He smiled ruefully at the tarlike substance in Ron’s cauldron. He passed over Ernie's navy concoction. Hermione's potion he gave an approving nod. Then he saw Harry's, and a look of incredulous delight spread over his face.
'The clear winner!' he cried to the dungeon.
— Harry Potter and the Half-Blood Prince, Chapter 9: "The Half-Blood Prince"