I spoke at the first systemd.conf recently.
This was an enjoyable experience, and I can highly recommend attending relevant conferences.
The value in conferences
Put names to faces of people you've never met in person before.
This adds a personal touch that makes future communication easier, since you get a feel for how approachable someone is, and they know you as someone they can talk to.
Some people's avatars look rather different to how they do in-person, and now I have a good idea of the people who I had been chatting to in
This is easier if the conference has social events arranged, as they often include alcohol, which can loosen the tongue sufficiently to get past the inhibitions and awkwardness of speaking to people you don't know so well.
Participate in the less open discussions that interested subgroups have.
Some large projects organise themselves into subgroups, aligned partially by interests, and partially by the organisational structures of where they work.
kdbusdevelopers don't normally hold the discussions of its development anywhere I frequent, but I maintain an interest in its development, so being able to hear how it was going was nice.
Discover common ground between different developers, to organise subgroups.
Systemd is normally consumed by downstream Linux distributions, rather than the users directly, who instead get it from their distribution, and there is an amount of work involved.
Because systemd.conf allowed all the maintainers from the distributions to get together in one room, they were able to organise a common working group, so that they could collaborate on downstream packaging, most importantly communal maintanence of a stable release.
Hear the gossip that you don't normally hear.
I couldn't possibly comment.
Attend or give talks.
Attending talks gives you the opportunity to understand the perspectives of others, in a manner structured to help you understand.
Giving talks is both the opportunity to share your perspective, and if, like me, you are uncomfortable with public speaking, the opportunity to step outside your comfort zone and expand it.
Getting involved in conferences
If your project is small, it is possible to organise a dev-room, in a community-driven conference, like FOSDEM, or a larger corporate conference where interests overlap, like systemd used to.
Medium-sized projects, like systemd, can arrange to have their own dedicated conference.
Large projects, like the Linux kernel or OpenStack have many, big, corporate conferences, in far away, expensive places, a few times a year.
There is usually some form of sponsorship available, for starving hackers to be able to attend conferences.
Big projects often have more expensive conferences, but usually balance this by having sponsorship more readily available.
Small projects may find sponsorship more difficult, so attendees will need to fund it themselves, which makes community conferences like FOSDEM more appropriate.
Call to action
If you're looking to attend a conference for some project that interests you, but you don't have a specific project in mind, you could look at the LWN.net Community Calendar.
If you are a member of a project and would like to get together with other developers or users, and there isn't a conference of its own for it, try to find a community conference that is near to most of the developers, and see if you can arrange for sponsorship for any more remote developers, so that you can have a get-together.