What is the use of ravage/damage potions?

I don't understand the use of Ravage/Damage effects in the potions. White Cap, for example, will Restore Magicka and Ravage Magicka... why would I make anything with this ingredient?

Solution 1:

Negative effects are for poisons, which can be applied to weapons. Bear in mind that not every effect of an ingredient is added to the potion/poison. Effects are only added when two ingredients with a common effect are added. So White Cap can cause Restore Majicka when added to ingredients that cause Restore Majicka. It can cause Ravage Majicka when added to ingredients that cause Ravage Majicka.

Solution 2:

You only get the effects from the ingredients that match. If you craft something with the White Cap that has Both of those effects then the potion will also have both of those effects. The trick to alchemy is to put together the ingredients in a way that will accomplish one of two things: make an expensive potion to sell regardless of its affects because you will never use them or put together the cheapest ingredients that will give you the desired effect (positive or negative) with out having too harsh of a consequence.

There is also a perk that you can get (100 rank in alchemy) that will allow you to make Pure potions, this should remove any adverse effect. I can not however speak to using that perk, I just read its description and that was it.