You should never leave your phone at home, especially if you ride solo. Should you get lost or hurt it is a valuable tool to have handy.
I recently discovered Strava. Before I was using MapMyRide to get an overall picture of the speed I was riding but with frequent stops its hard to know where and how to improve. Now I can quickly compare my own times on segments as well as check to see how I compare with my friends. I'll never be KOM but that's not important. Plus, the accuracy (or lack thereof) can give people a few minute head start if Strava puts them on the second switchback at the start or finish. Even with segments that don't have switchbacks there is still a 15-30 second margin for error. Strava is also a great way to discover new trails and routes. Some of the signage at small parks isn't great but with Strava you can check to see where others ride and plan your next visit.