What's the point of a custom ROM?

I have an international GS3. Rooted, with Nova launcher pro, which is fantastic. I have antutu installed to change my clock speed and keep it low most of the day when I only have very light use. I am a clear UBERRRRR NOOOOOOOOOB when it comes to this kinda stuff, but what is the benefit of a custom rom like Cyanogen mod? Especially nightly builds, which seem to be relentlessly buggy. The only benefit I see is for people who don't like their skins, like Sense or whatever, but I really dont have a problem with the Touchwiz menus and stuff, so I dont get anything there. Just convince me its worth installing a rom. Or at least explain the advantages an benefits to me.