Why do they do it?
Because some people say "YES"!
One cashier, at Wal-Mart, did try that 'Well Sir, everyone else got the agreement !'. I pointed out I'm not everyone else, and I didn't see the need.
I do know that some companies, the cashier gets part or most of the money, so it is for their benefit you pay it.
No, I don't buy maintenance agreements.