The main benefit for me (I have Aca 500 plus) is having HD on A500.
I'll ask you a question, that should give you an answer to your initial question.
Ok, I got Amiga 500 from Ebay, Joystick, mouse...
How can I play games? What are my options?
How can I transfer games from PC?
Very very hard.
These new cards makes that very very easy.