After reading my previous post, I wouldn't be surprised if you were asking yourself; "If there are scrolls whose creation doesn't require the destruction of material spell components, what use is potions? They store spells for use later in the same way as scrolls, but use up material components in the process when created. Wouldn't all wizards just create scrolls then?"
Well, for one thing, potions can be used by anyone, scrolls cannot. That may answer the question of why potions exist with scrolls as competition, but it doesn't answer why a character in the game would want to be an alchemist and create his or her own potions.
The answer is this: Potions can be created in bulk. While the creation of a potion will always destroy the material components used, an alchemist doesn't need to sacrifice a new batch of material components for every single dose of potion made. Alchemists can create potions using a cauldron to create dozens of doses of potion at a time.
Unfortunately, there is a bit of resource management involved here. If an alchemist only wants to make a single dose of potion, he or she must still sacrifice the material components. It's an all or nothing deal. Because of this, alchemists who aren't adventuring will usually create a large amount of their potion at once in order to save on material component costs, and then sell the leftovers.
What prevents alchemists in the field from carrying a cauldron around with them all the time and creating that many potions at once are five major limitations:
1: Weight and bulk. Potions need to be carried in something, and they need to be brewed in something. Carrying around the equipment required to create potions in bulk is heavy and awkward, and so is carrying around the potions that have been created.
2: Water. The wizard must have access to enough water to fill whatever container he or she is brewing the potions in, and it's unlikely that when potable water in a dungeon is precious the party will allow the wizard to use an entire bath's worth of water to create a hundred "cure minor wound" potions at once, saving on material component costs. The more skilled the alchemist however, the less water they need to use to create a single dose.
3: Time. It takes far less time to brew a single potion than it takes to brew a dozen or more. Brewing an entire cauldron at once can take hours or even days, days the party is unlikely to allow the alchemist to sit around doing practically nothing. There is also a point of diminishing returns, where the amount of money saved by making the potions all in one batch simply isn't worth the time it takes for the water to absorb the magical energy.
4: Danger. As I'm sure many of you have noticed by now, working with magic in Aarn is a volatile and dangerous process, especially if you don't know what you are doing. A cauldron that is not carefully tended to during its entire brewing time poses the risk of exploding catastrophically instead of nicely becoming a magical liquid.
5: Economics. Potion vendors in a city are unlikely to pay top dollar for potions created by an alchemist in the field. Unless the alchemist is their usual supplier, the shopkeeper will be very suspicious of the potions - they could have been created with tainted water! Or substandard material components. Or simply been created by an incompetent alchemist.
There is also the fact that while normal people will buy potions, the demand isn't high, and vendors are unlikely to want to act as a middle-man and add even more potions to their own already-overflowing stock. If an alchemist wishes to sell his bulk potions, he will either have to become a regular supplier for a vendor, or learn the skills of a vendor and sell his potions himself. Neither of these are viable options for the adventurer-on-the-go, though it can present itself as an excellent source of side-income during downtime between adventures.
Something to remember however, is that all skills in this game can be learned by all characters. A character can become a competent alchemist without dramaticly sacrificing battle prowess, other spellcasting, or whichever other skills a character wishes to learn. Jacks-of-all-trades are quite welcome in Aarn.