One person in every bike riding group should have this tool. It won't see much use hopefully, but every once in awhile it'll save a ride and/or prevent a long walk home. The chain tool specifically will come in handy. Chains, despite leaps and advances in flexibility and strength, can still snap or get twisted links. With one of these, you can temporarily shorten the chain and make it home.
Make it a habit to keep extra master links on every bike; otherwise you'll have to do it the old school way by pushing the pin almost all the way out, removing the bad link, and pushing it back in on the next good link. That'll work temporarily but it'll eventually break again.