People generally do need to develop better control than I regularly see demonstrated in commercial gyms, but trying to lift in slow motion has no inherent benefits. It's essentially just an awkward and inconsistent way to add intensity, when you could just do more reps or sets or weight or less rest etc.

You build muscle with progressive overload and calories. Details like lifting slower or faster aren't worth considering too much. Both are recommended in certain situations, but not so much for the purpose of muscle gain. You might like to do squats with a 5 second eccentric as someone who has poor control in the squat. But that's for technique improvement, not gaining any more muscle. Conversely you might try to squat as fast as possible for athletic purposes, even going so far as to measure barbell velocity which can inform capabilities in other exercises and sport specific movement. And they're not really missing out on much muscle gain, especially if they continue on with more muscle intensive exercises after their power focused movements.