Modern slot machines use random number generators (RNGs) to determine the outcome of each spin, so the result is completely determined by chance. The RNGs are constantly generating new sequences of numbers, even when the machine is not being played, so the timing of when you stop the reels has no effect on the outcome.
Pressing the stop button does not change the outcome. But it stops the spin and reveals the outcome in a shorter time frame. Many gamblers see the stop button as an attractive feature. Some studies have reported that it contributes to the illusion of control.