Maybe I'm the wrong person to write this. After all, I only go to one or two conferences a year because I can't quite afford to travel as much as some people do. I check out some local conferences and that's about it. However, I'm inspired to write this because I just got back from RailsCamp East Coast and it was AMAZING. It reminded me of the same reason I love Burlington Ruby Conference and my first PyCon. Conferences (or in the case of RailsCamp, a retreat) set aside a few days to learn some new things and also to spend some time with other developers, building relationships. In a way, it's the best networking you will ever do. Going to loads of meetups and making a passing acquaintance with a lot of people might do you some good. Really getting to know a few people over a few days will do you a lot of good.
I don't mean to dissuade anyone from going to meetups... I have made so many wonderful friends through PyLadies Boston that I would absolutely vouch for and that I've helped get jobs. However, those relationships have formed over the course of years and sometimes you don't have that much time. If you are a junior developer who is trying to get a job, one of the best things you could do is go to a conference and do some heavy-duty bonding. If you can do a talk, that's even better. Anything to show how interested you are in whatever language/field you want to work in. Going to conferences won't guarantee a job, but it will likely increase the number of people who are willing to recommend you to other employers and increase your chances of getting a better job.
If you are like me and a more experienced developer who is not looking for a job, conferences are still beneficial. I love my job, but I'm not going to be there for the rest of my career. When I am looking for a job, now I know even more awesome people who I would love to work with who also know me. This increases my chances of finding a job I actually enjoy since I have spent a significant amount of time with all of these people and have a better sense of who they are and what they value.
TL;DR - Go to conferences (and RailsCamp). They are fun and valuable.