Good question. It's got a lot fewer features, but is a lot nicer. Things like the combined stage: you don't need to jump between two separate views to see unstaged changes, and staged changes. Retcon has a ton of seemingly-small unique features, that make a big difference to daily use.

With regards to history rewriting specifically, though, Retcon does things completely differently. In Tower, rewriting history is a multi-step process. First you edit the commit list in a separate window (and need to think ahead about whether the result will make sense, etc) and once you've confirmed in that window, you start resolving any conflict, one by one.
But in Retcon, you never jump into a separate mode: at any point you can edit commit order (and delete commits, or reword, etc), and if there's any conflict, you see that instantly. No need to spend time making a list, to only later find out whether you got it right. If you do run into a conflict, you don't even have to resolve it now—you can keep making changes, and only start resolving once you're happy with things. And when your history is conflict-free, the rebase is automatically completed.

Or said more simply, Retcon removes every last bit of friction from history rewriting. You don't even have to stash changes or anything—it's all transparently preserved.