/////But what is it for?? I don't get it!////

I'm not sure who you're asking, Desi, but if it's me, the Circle of 5ths has many uses. It shows you all your available scales lined up such that all their relationships are visually demonstrated. It's fun to go through one and find the hidden relationships. As you can see from reading the posts here, there are as many ways to use it as they are musicians in the world.

When a scheme as this is truly useful, is when if can be employed by a technology that didn't exist until long after the scheme was developed. Here's an example:

How DJs use the Circle for mixing

The method of employing the Circle of 5ths here is totally different from what a classical musician might employ it for but it's equally useful either way. The Circle of 5ths is a thing of true beauty.