1. You need new friends.
If you are in your 30's or 40's and still single and childless, then you are in the same position I'm in. You have gravitated towards the few single gals that still exist and you find that all your married friends with children are all missing in action. It's not that they are not your friends anymore, you just don't have a lot in common anymore so essentially they have moved on and so have you. Meetup gives you the chance to meet new people who share a common interest.
2. You want to learn something new.
I recently attended a meetup group that focused on Spanish conversation. The group meets in Irvine once a week and there are native Spanish speakers and non-native speakers as well. I had an amazing time and learned that I need to practice my Spanish more often even though I'm a native speaker. I also met someone from Madrid, Spain and that was awesome because I had the chance to share with her all of my experiences when I studied abroad in Madrid over ten years ago. You might not want to learn Spanish but you may be interested in learning a new dance such as swing or salsa. All you have to do is a search for dancing groups in the Meetup website and choose a group.
3. You want to grow/expand your business.
I can't stress enough that you carry business cards with you wherever you go. This is especially true for Meetup groups where you can potentially meet a very important person that may want to work with your business. I attended a Christian Women's Business Network meeting recently and came across some very important women. It was interesting to see the various backgrounds the women had and what we all had in common was that we were all Christians. I would like to expand my blog so I passed out my business cards to everyone and I've received a couple of calls already. I'm also a tutor so I plan on updating my business cards soon. I've had coffee with one of the women already and I had a great time.
4. You want to explore new restaurants and/or communities.
Sometimes you will see that meetups are not very close to home. This shouldn't discourage you from trying it out. I recently attended a bipolar group in Fullerton and had no idea where the café was located. It turns out it was a cute little restaurant across from the Fullerton library. It was nice to try out a new restaurant for a change. I tend to stick to the same ones usually and don't really get out much. I would encourage you to try out new restaurants and/or new communities.
5. You want to start dating.
You never know who you could meet at the social outings. The great thing is that you already have something in common by attending the meetup group. I would encourage you to try different meetups and there are several out there specific to dating as well. I haven't tried the dating meetups yet but I might try it in the future.
I hope this list will help you make a better informed decision on what you can expect from www.meetup.com.