Jeff,
The AMA is broken up into 40-something districts for organizational purposes. The idea is to instill competition with local flavor while benefitting from the legitimacy and influence a nationwide association provides.
As for joining your district, which I believe is District 14, the only real reason for doing so is to participate in local AMA-sanctioned racing events. I'm not positive about the scene in Michigan, but in most places AMA races are the only ones of any significant turnout, prizes, quality and quantity around.
The cost of joining a district is usually around $20 per year and sometimes comes with its own monthly publication.
The contact for D-14 is Dick Ranney, who you can reach at (810) 636-2813.
James
00CR250