Okay, so let's say a Senate race is 90% about federal control and 10% about local pork.

It's not about the local pork. There's a lot of stakeholders in any state that may not care much about local pork, but they still have issues that concern them in Congress. Unlike Representatives, every single Senator is extremely powerful - especially on their committees, but on any issue within their caucus. That power has limits, of course - but it's significant, and it means that a lot of stakeholders in Texas (for example) will have a reason to be in the good graces of a Democratic officeholder for the first time in decades.

Again, that's enormously important for the nuts and bolts of ordinary party activities. Fundraising, candidate recruitment, outreach to organizations and businesses in the community. All those people and groups that had no reason to care about the Democratic party before, and no excuse to interact with the Democratic party before, now find themselves needing to cover that base. They'll find themselves more willing to support Democrats in local and Congressional races - because now those candidates will have the support of one of Texas' Senators, and that's important to curry favor with. They'll find themselves more willing to engage with Democratic elected officials. And for a Senator, they know that guy is going to be there for six years - longer than even a governor who manages a surprise win. It's the longest term office, which makes it a powerful base upon which to build.