Park isn't a subsitute for a handbrake - it's like leaving the car in gear, it's a precautionary measure. The park mechanism (it's basically just a lock, called a pawl, that engages with a gear), as well as the transmission, isn't designed to endure the full weight of the vehicle on a daily basis (i.e. if you were continually parking on steep slopes).
The parking mechanism in an autobox can be overcome fairly easily too, if someone hits it, so you need that additional braking from the handbrake.
.....plus there's the legal necessity for a parking brake, as well as a need to have that same system in reserve if the main brakes fail
Protip - if you want to ensure the maximum life from your automatic transmission, then do the following: when you come to a stop, first engage the handbrake then neutral. Once the vehicle is resting on the handbrake, then engage park. This means the transmission and parking pawl is not stressed, reducing wear and tear.