A non-trivial number of people do across the political spectrum. Political memoirs for instance are usually ghostwritten (outside of the first Obama book and maybe the Pete book given he was a nobody) and are only purchased in order to show support for the candidate. It's a team spirit thing.

Plus, I'm sure some have their own niches (satire, exposés like "Frankly we won this election" that have new reporting, etc) and many have their own historical value. I saw an ant-Tom DeLay book in the wild a few years ago well after his fall from grace and considered buying it just as a keepsake of the era.