They can be used for various purposes. For some, the reduced diameter provides for reduced drilling pressure and better final hole finish. They can also be ground with an flat or angled secondary cutting edge for simultaneously creating a through hole and counterbore or countersink.
Often used in production. You can drill a hole for a socket head cap screw where the screw goes thru plus the cap screw head can be countersunk at the same time.
Or you can drill and chamfer parts at one time. Time is money in production work. I have used hundreds of drills like that. There is a company in Downey Ca that is named Step Driils unlimited that grinds them to whatever size you need.