In political science we often model political actors as having fixed interests and positions, and then we try to figure out how they do or don't get their way. But there's actually more play in the joints of politics than that. Some people -- like Ronald Reagan! -- just switch teams entirely. More broadly, as we address in the book, entire parties switch their positions. If we want to understand politics, we need some way of understanding that process.
As I grow older, and have had more time to observe, I find the shifts in party positions fascinating and oddly opaque to most folks who are in the middle of them - perhaps this is one advantage to being part of neither major party. Some of the shifts are generational -- for example both parties have moved left on things like homosexuality and narcotics legalization. Some of the shifts have to do with who controls the White House -- the party in power tends to support executive power and military interventionism, while the opposition tends to oppose these things. Some of the shifts have to do with who controls intellectual institutions like college in the media -- the group in control of these institutions tends to be more open to first amendment restrictions, while the out-of-power group become desperate defenders of free speech (look how the campus free speech movement has shifted from the Left to the Right).
I would love to see a book on this covering the last 50 years.