Keep score of your tennis match and track corresponding fitness information (steps, distance, heart rate, burnt calories) at the same time. All matches are stored on your phone and you can go back an check scores and health/fitness related data for past matches.
You can keep/change the score of you match on the watch without having to carry the phone. Data will be sent to and stored on the phone. You can also just wear the watch and have a coach or partner keep the score on the phone app while the watch will collect and send health information to the phone where it is stored together with the match timeline.