I was thinking the same thing. I found this at Livestrong.com :
Knuckle, or fist, push-ups are identical to the traditional push-up but with one slight variation: your upper body is supported on your knuckles rather than on your palms. Knuckle push-ups are commonly performed by martial artists who want to strengthen their knuckles and wrists, thereby improving punching efficiency. There are additional benefits to doing knuckle push-ups.
Knuckle push-ups will help improve muscular stability in your arms, shoulders and core. Traditional push-ups are performed with an open palm, which is significantly larger than your fist. The decreased surface area of your knuckles requires the muscles in your upper body to work harder in order to maintain your balance.
Read more: What Are The Benefits Of Knuckle Pushups? | LIVESTRONG.COM