In piecemeal, potions are just two spells cast at once. The spell that is the effect, and the spell 'brew potion'
So you'll notice a few tweaks to how potions are commonly handled. One is that they expire. Potions have limits to how long they will last, and they lose potency as they go. You cannot simply stock up a backup of potions and leave them for ten years until you need them.
The extra difficulty in casting a spell, and the extra mana (a large amount) means that potions are prime candidates for material spell component use.
This is a good thing if you enjoy having even low level wizards being able to contribute in some manner, without destroying the medieval setting with a stockpile of magic items functioning as high technology. It allows wizards to plan for upcoming events without being able to just stockpile a potion for every need.