Dang - a good question!
- one scredriver to assist with the wiring
- one small phillips screwdriver to install the receptical
- one small-to-medium flathead screwdriver to install the cover plate
(I don't see the same number as above, but I agree with his point)
I can only surmise, that the flathead screws "look better" for the cover plates, and/or someone feels the "average homeowner" may not have a phillips head scredriver, or the "average homeowner" may noy be skillful enough to install a phillips screw without damaging the head,,,,,(just guesses, mind you).
Of course, the small bolts that install the receptical are typically a "mixed head", in that you can use either a flathead or phillips head screwdriver. If I have both handy, I use both. If I only have a flathead screwdriver handy, I use that.