There are a variety ofcordless screwdrivers out there and they're essentially the same. They accommodate a variety of bits and run on a rechargeable battery so once it's charged it's versitle and completley portable.
Turning screws by hand can be both time consuming and tiring. A power screwdriver gives you more torque or power to turn the screws and can obviously turn them.
Faster than you can by hand. they also give you more power to extract a screw that might be stuck and you can't get enough power to remove it by hand, but the power of a drill could strip the screw. The cordless driver is a nice medium.
These tools are compact so you can fit them into tight and awkward spaces that a drill won't get into or a manual screwdriver might be harder to turn.
A couple of tips, when you start a screw with a cordless screwdriver, hold the top of the screw against the driver and press it into place so that the threads bite into your project. Slowly start the screw and remove your fingers.
Also, make sure that you keep the driver level to be sure you drive the screw in straight and don't risk stripping it by trying to force it in at a slight angle. I'm Mrs. Fixit and it's just that simple.