Let's see , if a book gives a 100% bonus up to $500 do you not have to send them $500 of your own money ?

The only difference I see is a higher rollover.

By playing with the free money first you actually get the see the site first hand.

I know I have deposited at books and when I went to make my first bet I wished I had never deposited.

Hard to know if you like the software , or how fast they grade etc.....