What is the purpose of Cache?

Not trying to be ignorant, but I can’t determine the purpose of Cache. I ask because we are optimizing some file loading processes in our app.

I have examined Cache and see it holds a collection of already fetched files.
I also examined FileLoader and see it simply fetches a file and then places into Cache.

Finally I looked at documentation and I see Cache is a cache module that holds the response from each request made through this loader, so each file is requested once.

So it seems that the purpose of Cache is that each file is HTTP requested once. If I have cache headers set on my resources, and my browser honors the cache policy, then is Three.js Cache necessary?

Thank you for the insight.