I commute using, through the year, about a dozen routes so I've created them as routes in MapMyRide and when I get to work or return home, I record the ride and the time, if I've timed it. I find that with my phone's GPS (likely the same with a Garmin) that there are minor variations in my actual riding: on a mapped 24.97km route, my GPS may record it as 24.54km or 25.36km and I don't care about GPS inaccuracies or minor deviations in my route (cutting a corner closer when no cars are around, etc).

The other advantage IMO is that I can plan out a new route (commute or fitness ride) using the map and know the distance before I go out. I always ride alone (not that I always want to, I just don't have any cycling friends).