The touchpad is the reason, and it is not a gimmick — a large number of PlayStation games use swipes and taps on it for commands, inventory and map control, which is information a normal pad simply cannot send. The clickable pad also gives a second set of inputs for the same buttons. The built-in speaker handles system and social sounds without a headset, and the light bar reports player number.
The PlayStation pad, and the one that goes with a PS4 when that is what is being played.
It is an older design and the touchpad has become an ergonomic liability — holding it with a thumb is awkward for long sessions and the capacitive surface stops registering when it gets sweaty. Battery life is around 19 hours, well short of the Xbox pad. The original model reports 1000Hz polling only via USB in later revisions, and Bluetooth polling is lower.