I'll leave you to read Craig's answer on his blog, but it's a very reasonable question. Craig mainly asks the question from the point of view of the attendee, but it's just as valid (perhaps even more so) to ask it from the organiser's point of view.
On the negative side of the ledger, it's a lot of work. Just ask Greg or Mitch or Rocky or Graham or any of the 30-odd User Group leads around the country. You're going to be able to make more money, or spend more time with the kids or whatever if you don't do it.
On the positive side, you're doing three things:
- You're providing a service. This means that people in the community look up to you as the person who runs/ran <insert event or user group here>. Your name will be associated with the event or group and you'll most likely get referrals based on that recognition
- You're building a community. As someone who has benefited a great deal from various tech communities over my career, I can personally vouch for the value of community. Apart from anything else, having an expanded user base will at least assist in future-proofing your career.
- You're Learning. As the organiser of an event or group, it's your call as to who presents and on what. If there's a particular topic you'd like to see more of, organise for an expert to come and present and there you go.
As I think I've said before, It's all about community.