There is no value generation in trading stocks either beyond IPO, as stocks are then simply second-hand markets and can be completely irrational. The main value is just like the lottery : entertainment, excitment.

There is value beyond IPO- companies continue to sell ("issue") shares on the public market as a way of generating cash. They also buy back shares when with excess cash, usually when the market value is low but sometimes, irrationally IMO, when it is high. They also use this as an incentive-backed compensation pool. The relative value of these is debatable, but I don't think it is correct that post-IPO public shares are just traded between third-parties making short-term bets. Happy to be corrected.

There is strong signal that companies then listen to and act on. The market serves (whether you like it or not) as the final board of executives.