They also work great for timing planks and cobras, where for me a big part of the problem is spaciness/boredom.

I either go too short on rest periods because I got bored or too long because I spaced out and forgot to watch the clock. I've been working out before going to work, which means I need to get up in time to exercise, eat, and shower, and having timed workouts make this possible. I only just started with weights, so I'm not sure if I'll need to drag myself up earlier to get everything in yet. But having music and knowing about what time it is based on what song is playing is incredibly helpful.