I've heard the pay for NFL cheerleaders is ridiculously
  low. So I wonder what the benefits of being one are.

Just because a job has low pay, limited opportunities for advancement and poor job security, doesn't mean there won't be people willing to do it.

I mean, if you've got a choice between being a shop clerk for minimum wage or being a cheerleader for 2% above minimum wage, you might choose the latter even though it's low-paid.