Do you think it is important before you start sparring or just spar and shadow box when you start boxing?
Very, bags have return punching air don't/ On the bag you can punch like a wild man for hours, in thin air, you realize you can't punch too hard without throwing your entire body off balance. You arms get tired if you are not use to shadow boxing. It teaches you internal balance to throw hard punches without falling over if you miss.
It is where you train your brain to think boxing. By constantly envisioning an opponent and what he is trying to do to you, the moves he is making and the punches he is trying to land, and, of course, by then figuring what you are going to do in return while avoiding his attacks and countering with your own. It doesn't do you much good at all if you just kind of stand there and throw half-ass punches at an imaginary punching bag that is just there to be (pretend) hit.
:good Shadowboxing is probably the best thing you can do other than spar. And if you aren't imagining an opponent (a good one) then you're wasting your time. Shadowbox all the time, it's fun.
Probably the most important training tool. Shadowbox in the ring, in front of a mirror, while running, in the pool --- all good. You work your legs, punches, rhythm, balance. Bob, weave, changing weight and punching. Work on moving your head, slipping and countering. Work on your feints. Work on breathing & relaxing while being busy. There is a lot more.
I think its the most important exercise besides sparring. I could see a boxer not using the speed bag, double end bag, even the heavy bag, and still have success in the ring. I don't think there has ever been a boxer who didn't shadow box lol.