Here are some possible reasons. For everyone it's different.
To live a more awake and authentic life.
Maybe because on some level you really need the world to be much better than it is! Perhaps you want such a world for your children, or for future generations.
To have a structure that helps you feel like you're moving forward on a weekly basis.
To get one or two people who will support, encourage and sometimes challenge you. To get some affirmation for your sincere efforts.
To offset or neutralize negative and despair. ("It's better to light a single candle, than curse the darkness.") To create hope in yourself.
To feel like you're part of a movement that is making a difference in the world. [Once we grow!]
To give back (in appreciation for the people who've helped you.)
Rather than to always be passively affected by events, you become someone who has an impact, who changes things.
Because you have natural empathy for others who are suffering, and you naturally want to help change their situation.
Regular effort usually makes you stronger and more resilient, but empowering and supporting others makes others stronger and more resilient at the same time!
A bonus reason: To prank and mystify your friends and family, if it's not something that occurs to you naturally. (Just don't tell them why you're doing these good actions.)