Crafting potions and elixirs that will improve your performance while you're doing strenuous activities is the most important thing, as it's what makes the Water-filled Glass Flask a fundamental crafting ingredient since your character depends on the use of alchemy to boost its stats if their gear is not enough on its own. Some foods and drinks can also help with that, so making Purified Water with Water-filled Glass Flasks is something you should also try if you wish to level up your cooking and brewing skills. Various potions, elixirs, foods, and drinks can randomly spawn inside loot chests and enemy corpses: if you loot them, you're skipping the whole crafting process, including making Water-filled Glass Flasks, but remember that it's necessary to craft these sometimes, in case you plan to get the mentioned things more effectively and in large quantities.