Large checks are cheaper to handle than an equivalent amount of cash. Most people won't just toss $10k in the back of their car and drive to the bank, but would think nothing of taking a $10k check to the bank.
Also, there is less of a need for a receipt when paying by check. If I pay someone by check, my bank has a record of it. If I pay them by cash, I better get (and keep) that receipt in the case of a later non-payment dispute.
