A joint patent for this design is held by the Phillips Screw Company and the American Screw Company. The purpose of this screw is to increase the torque which can be applied before the screw and driver interface "slips".
A regular screw driver can be used to turn Pozi screws however a Pozi driver should not be used to turn non-pozi Phillips screws. The four extra spline will prevent the pozi driver from sitting properly with in the screw and cause rounding of the four Phillips slots.
The term Pozi Drive is often confused with Posi-Trac. Posi Trac is General Motor Companies term for a limited slip differential. LSD not the screws on your skis.
The American Screw Company...Imagine all the marketing material you can come up with for those guys!