Wisdom teeth are ancestral vestiges - sort of like an appendix, or a "vestigial tail" (an extra bit of cartilege at the base of your coccyx - aka tailbone). If yours is grinding against the tooth on the other jaw, then your teeth are malaligned. Wisdom teeth serve no function in the modern human mouth. They're "extra" teeth, and many people never grow them at all. Many people who -do- grow them, don't grow all 4. And many who do grow all 4, end up having to have at least 1, if not all 4, pulled, because they are pushing the -rest- of the teeth (which are necessary), forward in the mouth, crowding them and causing malalignment.
You don't need your wisdom tooth. However, if there's nothing wrong with it, and it's not crowding any other teeth, then you don't need to have it pulled. It can stay there indefinitely, until it starts pushing your other teeth forward, or until it starts to decay. At that point, it's cheaper to just pull it, and since it's an unneccessary tooth in the first place, there's no reason why you'd -want- to save it with a root canal or crown.
Wisdom teeth should be either a) left alone if they're not troublesome, or b) extracted.