A deposit is there to prevent a dealer loosing money...
For example say you see a car you want.. You tell the dealer you want it, 2 mins later someone else comes along.. Dealer says its sold, and you never come back. Dealer loses a sale.
So a deposit secures it for you and covers him incase you just change your mind. Although toi be fair thats a big deposit, I usually stick £50 - £200 on a car depending on its value to hold it til I can get the funds togethe!