It's not redundant, and there aren't just two types of screwdrivers. Each screw design has a specific purpose:

Flat head screws are easy to manufacture by hand, so they were the standard for centuries. The first screwdrivers as we know them were invented around 1800.
Phillips head screws were invented as a self-centering alternative to flat head screws. The design was invented by John P. Thompson, who sold his patent to Henry F. Phillips. Phillips got the design off the ground by getting it adopted by auto manufacturers.
The Robertson/square drive was invented by a Canadian, and is heavily used there. Eventually, its use spread because the screw naturally seats on the bit, and it's easy to cold press into shape. These are commonly used for deck screws and woodworking. Variations add more points (double and triple square) so there are more angles for the bit to slide into the screw. These are used by automotive companies.
Hex/Allen screws and bolts are the result of numerous attempts at making an internal-wrenching screw. These were adopted due to the low cost of manufacturing a hex wrench combined with safety concerns. The design allows the screw to be flush with whatever it's in, so there's no possibility of catching on it. They're common on machinery and bicycles.
Of course, there are a lot more designs out there, including security bits, spanner head, etc. Each exist because they fill a specific role.