A power drill/driver gets its name from its dual purposes. Sometimes, you use the tool as much for driving as for drilling, particularly when pre-drilling pilot holes on hard materials or using a countersunk bit to recess the screw heads. How much time is wasted as you load the drill bit into the chuck, drill a hole, remove the drill bit, load a driver bit, drive in the screw, reverse and then do it all over again?
Maybe the process is not too bad if you only have a few screws to drive or your project lets you work on an assembly line to do all the drilling, change bits, and then do all the driving. Maybe you have two drills and dedicate one for drilling and one for driving...or maybe you should investigate the Twist-Lok Drill and Drive System to learn how easily you can lessen the nuisance factor of repeatedly changing bits.
This system eliminates the need to remove the drill bit and instead allows a bit driver to be slipped directly over it. The video demonstrates this very clearly.