cpa network signed

I am new in CPA market so I need best CPA site for the beginner. I found lots of CPA network but can't understand what is good to me So, I need best CPA networking sites for beginners.

I don't get it. Why do people sign up for multiple CPA networks when you can just stick and concentrate on one? I see some people with 6++ different networks. Don't they all do the same thing - pay you after the survey is done? Why sign up for so many o.O