What are alliances for?
There are two kinds of alliances, those in the forums and those in-game.
Those in the forums (look for Forums > Alliances > [alliance name]) are more strategic cooperations. They tend to have stricter admissions policies and require a certain standing before accepting people. They tend to operate in only a few specific worlds (such as how Dynasty World operates in RDelta), but have much closer cooperation in comparison to the strictly in-game alliances. Expect teamwork, cooperation, and tips and tricks to come your way with these alliances, but know that you need the experience to join some (especially those like Unitedwings, Dynasty World, or Azure). There will also be some policies like anti-competition and cooperation involved with joining one of these alliances.
Those in-game tend to do much less. If you look under "Reputation," 10% of your possible reputation comes from alliance destinations served. If you feel as if you'll be accepted in one (and acceptance rates are usually high), then join one. You should (depending on the size of the alliance) gain a few reputation points. Some alliances in-game are connected together, either via threads in the alliance forum or external methods such as Slack or Discord. For the most part, however, many treat in-game alliances as ways to gain reputation and have little in terms of actual cooperation.