For the first few years I started cycling, I didn't have a cycling computer that could measure and display cadence. This app helped me get a general idea of what my cadence was. By tapping the button that says "or tap the tempo here!" I could measure my cadence. Afterwards, I would set the beats-per-minute to match my cadence, then leave the metronome running throughout my ride. If my legs moved faster or slower than the beat, I knew my cadence was higher or lower than the setpoint.
This app is also useful for strength training, especially for tempo work. Having this beeping in the background helps with precisely keeping the pace of the eccentric and/or concentric portions of the lift consistent.