Know who closely watch off year local elections? Members of Congress.
Most of them came up the ranks though local elections. Almost all of them raise a significant amount of money inside of their districts. All of them have offices located throughout those districts. They are political animals with their ears to the ground. Congress critters notice local voting trends the way weather forecasters notice barometric pressure changes. The way many of them vote on a number of controversial issues when Congress reconvenes next January will strongly be effected by the omens they read from this November's results. Not just who wins and who loses, but more innocuous data too, like percentages of voter turnout. And whether or not younger voters are actually voting in an off year election this time.
Maybe the most consequential thing any of us can actually do to influence national political policy, prior to the Congressional mid term elections in 2018, is to help drive voter turn out in 2017.