It allows the hiring company to keep an applicant from straying, without having to commit the resources to the checks until after you know the employee will accept the position.
Except that it doesn't keep the applicant from straying. If he gets a better offer from the time he accepts and the other company gives him a firm start date of next Monday while the first company still has two more weeks to go of dicking around with their internal paperwork, odds are the first company is going to get a phone call rescinding the acceptance and thanking them for their time, especially if they pay in arrears. On the flip side, I've also seen people accept an offer, start work, and then leave after two weeks when the job they really wanted with another company is offered.