The reason to vote within your registered precinct is partly because ballots may differ with local issues and partly just historical structure of how votes are

The reason to vote within your registered precinct is partly because ballots may differ with local issues and partly just historical structure of how votes are tallied and reported hierarchically. The local officials are responsible for maintaining voter rolls just as they maintain other vital statistics. You can vote by mail even as an expat living overseas. You register via the last address you had in the US and end up getting a more limited overseas ballot that only covers federal elections (and possibly state-wide elections if registered in some states?).