Potions are good, not neccessarily vital, to have. Some potions render you invincible, some heal HP, some MP. Some transform you into various things. Some poison you, some cure the poison. A select few are ever found. Mana and vitality potions can be purchased at any consumnables trader. The consumnables traders in the main city and the starting area also sell potions that increase defense values. Potions are expensive depending on what you want, and there are very few that are really cheap.