Being able to use a connected game controller to control in game aspects such as camera controls, ride ons, selecting left right forward at intersections, power ups etc.
Why not just use a keyboard or phone? Because a game controller is designed to easily fit in one’s hands and I don’t want sweat getting on my nice keyboard! It’s something I can easily have draped over my handelbars for instance and grab when required, as opposed to hunting for the correct key on a keyboard which is also much more difficult to balance on my handlebars! Plus, phones are hard to use with sweaty fingers and I often am doing other things on my phone like for instance watching videos instead of always having the Zwift companion app open.