When I hit the wall I try to damage the wall by making the paint chip or making the particle board splinter. If there isn't any paint, I focus on breaking strings. I think of wall practice as hitting drives for reps at an indoor range; trying to train that muscle coordination, explosiveness. This is a bad idea if your technique is flawed in a way that'll lead to injury, or if you're trying to become languid like @sureshs. But if you're training to hit hard, it's a great way to make that full speed swing more automatic and less efforted later. It's literally exercise.
As far as calculating ballistics after the ball hits the wall, I have no idea why anyone would care. If you don't know when you've hit the ball cleanly and with good shape, you shouldn't be worried about reducing the amount of time you spend on a court. Get some lessons or hit more or whatever, but don't tell people that they're wasting time. Armchair warrior neckbeards.
Anything with a racquet in your hand is additive. OP is spending his time well.
Yes, that is one reason I am using during warm-up or kind of cardio workout. Hitting hard and try to move my feet (not good at that though yet).
I hope my techniques is good enough now, to not screw it up or injure myself
Second use case which makes sense for me is slower controlled hitting to a target as @ubercat mentioned above (great reminder tip), working on precision and consistency.