That's one aspect that's relevant once you're done, the other one relevant during the workout is that you can set different targets for different parts of your workout, e.g. you want to be at 100 bpm during your warmup, 160 bpm during a 4min interval, then rest 2min at 120bpm, things like that. If you don't strictly follow the program you initially planned (e.g. you want to have as much rest as you feel you need) then you'd have to interact with your watch.