It serves the anti-Southerners who want to tar the entire Southern people with Confederate ideology, and it serves the anti-Northerners who want to portray the North as cultural imperialists.
That's why learning the true facts can be so important. Then the Civil War becomes a poor vehicle for regional chauvinism. Get that out of the way, we can argue over more relevant differences such as grits and auto racing.
Mythic thinking likes these big "North vs. South" abstractions. They're easier to remember and they make things tidy. When you really look closely, it's an awful lot more complicated than that. You run into ambitious politicians with their own agendas.
I suspect people are looking for a symbol of their own frustration or anger with Washington or "the Establishment" so they seize on the Confederate Battle Flag, which does make more of an impression than, say, the "Don't Tread on Me" banner. Having chosen the flag, they reinterpret history to justify their choice.