Why is music often played when people are doing physical exercise?

I'd say music helps people push through the physical discomfort of exercise by distracting their focus. When I was training for my first half marathon, I noticed how much easier it was to keep running when I had upbeat songs playing - they sort of covered up the tiredness in my legs. The rhythm also helps synchronize movements, like when I see people at my Shanghai gym matching their treadmill pace to the beat. That being said, some serious runners prefer no music at all to stay fully aware of their body's signals.