Two reasons. The most important reason is if you keep the money for 30/60/90 days, it's like a interest-free loan for the company. The stronger your position is, the longer you can hold on to the money (see walmart). It's how virtually everyone does it.

Second reason, "A bank transfer is fast and trivial to do" - I'm guessing you've never done work for or been around a large enterprise or you would not be saying that. Usually it's something like "The spending committee has to approve, then Richard in accounts receivable is the only one who can sign off on the check and he's on vacation til June/"