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Local boards (when applicable) allow for discussions to take place in your own language, which often allows the poster to express himself in terms that better convey what he wants to say or ask than in English. Obviously there are many here that are multilingual and have no issue whatsoever on the general (English) boards, but I gather that there is still a strong base of people that feel more comfortable engaging in their own language, besides some sets of people that struggle with English itself.
One other relevant aspect I find is that many of the topics that surge on a local board are country specific, and being so, are probably better discussed amongst those that have boots (or roots) on the ground, where a more refined knowledge (and interest) of the matter at hand may be displayed.
Bear in mind that local boards are not necessarily relatable to a single country, but rather some of them agglutinate a bunch of countries through a common language (and perhaps culture), which has its perks as well.
As mentioned, posting there is a personal optional preference, and I’m pretty sure that there are many on end that decide not engage on their local boards for whatever reasons, which is obviously fine (though I’m pretty sure many local boards would welcome more participants).