One major advantage of going to a conference is that for the three days or so that your'e there you can focus exclusively on what's happening in your field. Yes, with online access you can read papers instantly, you can access preprint servers, etc., but the act of going to a conference, in my opinion gives you the ability to shut out your other responsibilities for a few days and focus. And that's pure gold.
Another comes through informal conversation. You can chat with people that are working on stuff that's similar to what you're doing and get the "story behind the story." How confident are they really with what they're presenting? What are they working on next? What resources do they have available to them or what are the planning to get? What have they tried that wasn't successful that they never bothered to publish and thus what can you avoid wasting time on?
Informal conversation with people you may not otherwise associate with is one of the cornerstones of creativity. So the story goes, when Steven Jobs set up Pixar Studios, he put all the washrooms in the centre of the facility, and this was supposed to trigger those spontaneous conversations that give both people new ideas.
Since I am a
medical physicist, I have relationships with various commercial companies. Conferences are an opportunity to find out about what the next release of software is going to contain, or what's so great about the three devices I'm considering for purchase in our new centre. It gives you an oppotunity to talk with the developers and trainers and get your hands on the stuff that's new.
Finally, in my field networking at conferences is huge. As a prospective candidate, they give you an opportunity to meet the different people who work in a facility, to find out by work of mouth who might be hiring in six months when you plan on gaduating, and to present yourself informally. Often hiring decisions can come down to how much the people on the hiring committee like you and if you've gone to a "night out" with them, it can make the decision that much easier. As a prospective employer, you get the same opportunities from the other end.