It's basically just a fancy way of saying 'beta' - they're unfinished dictionaries which are still being debugged (problems like missing / corrupted entries, bad character set conversions, etc) but which we feel are usable enough in their current form to be worth getting for many people. The "early access" part lasts until we're satisfied that they're in good enough condition to release officially - usually a few months. So the advantage is that you get it earlier, the disadvantage is that you have to put up with those bugs until we release a finished version.