Fevers are in theory useful but in practice dont make that much of a difference to the course of most illness.

Most fevers can be left untreated, as it is rare for the body to generate a fever high enough to be harmful.

However, fever is very uncomfortable, and can interfere with other things that are important to healing - hydration being the big one - lots of folks, especially kids, will throw up and have trouble drinking when they have a fever, so bringing the fever down can help with a vital part of staying alive.

Treat fever if the symptoms of the fever itself are troublesome, otherwise generally fine to let it do its thing.