Why
Installers can do far more than copy files: add background services with full system rights, schedule daily updaters, start with Windows, open the firewall. Windows doesn’t tell you, and uninstalling often leaves some of it behind.
What was hard
Telling the app’s changes apart from Windows’ own, which keeps working the whole time. A file’s place in the Windows folder proved nothing: well-known apps put their own drivers and services there, and Microsoft signs other companies’ drivers, so only Windows’ own signature counts. Many apps also add themselves to startup only the first time they open, so idiff asks you to open the app once before it looks again.
What it won’t do
It only reads. It never changes, cleans or removes anything, and it runs only the one command you give it.