Why do I need compressors, limiters, harmonizers, etc?

HELP ME!! I'm looking to go into electronic dance music production and I'm very green. I understand the softwares such as ableton, reason, etc. and the whole purpose of them. But when I look at producers such as Deadmau5, ATB, and big name producers I can't help but get intimidate by all the hardware they have laying around, especially the racks just stuffed with compressors and limiters or what have you? I don't really understand why those are a must. I mean I've played around with ableton for example and it already has built in limiters and compressors, is it just not enough or something?? Please help, I'm dying to learn.