Lets be honest: conferences exist to waste time. Nothing productive gets done on conferences and best thing you can take our from them is to socialize with like-minded people.
In my view, as someone who contributes to free software as my job (though the rest of this is somewhat related to proprietary developers too), conferences are a great opportunity to have IRL discussions over current development topics as well as meet people from other free software communities (who might work for proprietary software companies). And arguably that is getting work done (and I have written patches side-by-side with someone while at conferences quite a few times).
Having connections in your field, as well as being well-known by your field (perhaps by submitting talks to conferences) is very useful for both you and your employer. In most cases, a lack of conference attendance by a company is seen as a "bad smell" in some communities (it indicates that they don't have an interest in the community around something they use -- maybe because they think that interacting with their community through conferences is a waste of time). It also potentially gives you access to more recruits, because people looking for work often attend conferences to expand their connections.
Not to mention that most medium-and-larger companies can spare a few thousand dollars a few times a year to send their employees to a conference to act as both a marketing tool and as a form of training at conferences. I'd bet that medium-sized companies spend more on stationary and tea+coffee supplies yearly.
> They are a waste of your employer’s time...
Think about why tech companies provide ping-pong tables and nap-chambers (obviously unproductive) and yet Wal-Mart isn't sending its warehouse workers to warehousing industry conferences (presumably at least somewhat productive).
1. Decide up front what sessions I am going to attend. All of these things should be stuff that I know very little about or take my knowledge from 200 to 300. Don't go to sessions that you know the answer to. Be super selective on sessions as well: skip vendor shills and affirmative action placements.
2. Look AND SCHEDULE casual between-session coffee meetings with folks I know are attending. This both deepens my relationships with them and you also tend to meet people most effectively this way that happen to be with that person.
3. Go to birds of a feather (if possible). People are much more likely to want to meet other people at these things.
4. Ask everyone you can "How their conference is going?" and "What are you working on in this space?". I make it a goal to ask at least ten people this a day between the coffee station lines, lunch, happy hour. At a great conference people love talking about what they are passionate about.
5. Drop out of late night events and drinking sessions if you are just there for the music or the beer and not getting knowledge or connections out of it. This is business, not party time.
If you can't do these things, yes, it is a waste of time.
But if you can, I find that I can leverage all of the info and contacts out of this relentlessly at my $DAY_JOB to get a lot done the rest of the year.
In practice, I just don't seem to get round to watching them. Going to the conference means watching a whole bunch of them with no distractions. Very useful.
It's just a PITA to get Legal approval to give presentations.
Some of the best things that happened in my career spawned from networking at good conferences.
Depends on the conference, I suppose, but most I've been to include a healthy dose of that.
Certainly writing production code for work is rare, but for fun? Lots of it gets written there.
steveklabnik in another comment: "A ton of open source project work gets done at conferences".
Basically any statement to the effect of "everybody does/nobody does X at conferences" is almost certainly false.
It is possible to go to one of those in waste a lot of time, and party. For some people that's a once-a-year relief valve nothing wrong with that.
It's also possible to go have a lot of really meaningful, relevant conversations with people that are valuable to you.
That is the point of conferences. Meeting like minded people and having interesting conversations.