I don't necessarily mean you have to be sitting right in the middle of the sportsbook, I just mean your ease of access to your funds is much more convenient. You walk into the book and collect your winnings easily. We're dealing with offshore guys. They play all sorts of tricks that sometimes leave you without access to your funds for weeks at a time. Rollovers are basically requirements that are issued before you can cash out.
For example, If I deposit at a certain book, they issue you a rollover of 10x. I deposit $100 and get a $100 bonus, which is basically a free $100 given to me to play with.
$200 times 10 = $2000 of rollover. I essentially have to wager $2000 in total money before I can cash out.
There's just more incentive to play with the money more. However, there are some books without these requirements like a Bovada or a 5Dimes (they do have a deposit rollover) where you can easily pull hit and runs, but still, it's inconvenient. I hope I live to see a day where I can go to a local sportsbook and play and collect without having to travel to Vegas, but that's probably not happening in my life time.