The democratic party in my state puts money and campaign assistance behind smaller local races (congressional, state legislature, and small local races) based on primary numbers. My county has about 0.7x dem primary voters vs. our rep primary voters. As we're finally turning blue enough that we might win a race here and there, we're getting a lot more assistance from the larger dem strongholds in the state. My congressional rep is doing more events in my county instead of focusing on the bigger cities in their district. It's all a snowball effect. More engagement by dems in the primaries means more focus on campaigns in my area means bigger dem voter turnout in my area, and the cycle builds upon itself. If I vote republican in the primaries to be a spoiler, the dem party just sees my county shifting further right. Plus, my primary vote actually helps decide on my dem candidates in my region. It's a whole lot more than just one office.