A BMA is exactly what its name implies it is: an association of a city's bike messengers. They serve various purposes in the multiple cities that have formed them, the most important of which are outlined below. What exact form they take is up to the individual city.
Three very basic reasons.
First, because the network that has started to spring out of the CMWC/ECMC/NACMC/HPR events needs more structure. Networking word-of-mouth is great, but think of the flow of information if whole organized local associations were involved, and the potential they would hold for rapid redistribution of such information.
If you've got a local organization putting out a newsletter or zine and organizing events, etc, they serve as an ready-made conduit for relaying that information to the rest of the worldwide messenger community, and for receiving information from other BMAs. Even considering something as small-scale as finding places for visiting messengers from out-of-town to stay, would it be easier for them to call each person they knew in that city one by one to find it, or simply call a central body? And with more and more messengers on-line, and other BMAs starting web sites and the like, communication can now be faster and more convinient than ever.
Especially now with messengers travelling and working in foreign cities with increasing regularity, the networks to facilitate for these adventures fall neatly into the capabilities of a BMA. Say you want to work a few weeks in Hamburg while passing through there - would it be easier to call a BMA and get the straight dope quick, or to try and track down that one elusive person who might get you a job? Even though these sorts of things are fairly easily accomplished as things stand, wouldn't it be great to be able to have a little more planning capability?
Say you've got an issue with your local police (see also below), and you dont know quite how to deal with it? Most likely there'll be a BMA in another city with some archived advice for you. How about if you're planning to host CMWC some year - why start a whole race organization from the ground up, only to dissolve it when things are over? With a BMA, that organization is already there, and can continue work long after the event is over, with that much more organizational skill, and that much more of a rapport with the general public.
Second, because messengers in many cities are beginning to come under pressure from legal, political and labor issues, and attempting to understand, fight, or take part in these issues as individuals does not hold nearly as much weight as a unified front such as an organization would provide.
Put simply, if you want to be able to have a voice in the issues that traditionally messengers have collectively sat on their asses and bitched about (and accomplished little), you either need to have one utterly devoted and loud-mouthed individual willing to put themself on the line for the whole community just out of the goodness of their heart, or a collective force of your local messengers working for their common good. While the latter is more work to pull into shape, the former is more likely to burn out or get resentful.
Associations also eliminate doubt as to whether you're speaking for the whole community (or at least the majority). Beyond that, they provide for the division of labor, allowing members to concentrate more intently on the tasks they need to accomplish towards specific goals. If you really want to make an impression on your local politicians about the problems we as messengers face on their streets, or want to take a stand against the stereotypes your local media bombards their readership with, what's going to be more effective, one person, standing alone against it, or a BMA, making noise and standing up for our trade as a group?
Thirdly, and on a purely practical point, BMAs are organizations, which allow for things such as group rate health insurance, obtaining wholesale licenses (so you can buy bike parts cheap) and make possible the renting of support spaces. All that, of course, is pretty self-explanatory.
In other words, given the challenges many of us face as messengers, we cannot afford not to stand together, whether locally or globally, and what better way to start than a local BMA?