It's safer to send checks in the mail than cash. They can be wrapped in paper and obscured, and further hidden by using opaque safety envelopes. They are also less of a draw for thieves than cash.
As most people write checks, they tend to record it in the register. With credit or debit card purchases, they may not take a note. Duplicate checks also help with record-keeping.
It's quick and easy to prove that a payment was made with checks. You need only show the canceled check or image from an online banking page. After cashing the check, there's nothing else to pay, unlike credit cards that quickly grow a balance.
Checks act like cash in the store, but in many cases take a day or two to be cashed. However, some vendors do present the checks like a debit card, meaning they clear almost immediately.
Many places, including online vendors, do not accept checks. Writing checks is a little more time-consuming than swiping a debit card. While some checking accounts charge fees, some banks (like Chase) offer free checking accounts.