Things like mayoral, councilor, and other positions highlight the fact that a good deal of politics is local. The scale of something like city gentrification isn't controlled by a federal party, it's decided at city hall. Even though a good state or federal representative will look to improve the lives of their constituents, local politics is where direct action occurs most effectively against things like gentrification, city transport limitations, homelessness, and other marginalizing strategies against people who deserve better. "The Democratic Party" as a sweeping target doesn't fit neatly with local specificity. And so if a local councilor is a Democrat, but does not work to help people who need it, that doesn't actually say anything about a broader, general Democratic platform--it says that they, in particular, are just bad representatives.