I always bring my smartphone on a ride for 2 reasons:
1. Any sort of emergency that needs 911, or less extreme emergencies where I can call a cab or a friend to pick me up (my bike frame broke once while I was riding - that's not the sort of thing you can bring repair equipment for lol). I've never had to call 911, and was able to walk home that one time with the frame breaking (rusted out at the downtube), but a cell phone is a device that let's you communicate with anyone from (almost) anywhere - I think it would be stupid not to bring it. If you prefer to leave it off and only turn it on for emergencies that's fine with me to, but it's the best emergency device you could have with you.
2. It makes my rides around new places FAR more relaxing knowing that if I get lost, I can pull out my cell phone and bring up google maps and find out exactly where I am and figure out how to get back.
I don't use my cell phone almost at all when riding, but having it as a backup makes my rides more relaxing.