Leaving the bets open while the ball is spinning causes a lot of players who think they can time the ball (or as another commenter said, believe the dealer might be cheating) to bet and presumably yields higher profits.

There are other countermeasures developed nowadays, such as Cammegh's RRS (Random Rotor Speed): https://www.cammegh.com/our-products/roulette-wheels/mercury... - essentially after bets are closed the wheel is able to ever-so-slightly slow down at random times, throwing off any prior calculation.