It's basically a reinstall where the user's configuration and preferences are preserved. As to why - perhaps a file got corrupted (rare, these days). But often

It's basically a reinstall where the user's configuration and preferences are preserved. As to why - perhaps a file got corrupted (rare, these days). But often what happens is another program overwrites a shared file to one that is incompatible with your application. Or the user watches a video that tells them about this secret hack that makes your computer run faster... Story time regarding DLL naming from the old days - Windows 95 would only keep one copy of a DLL in memory (the days when having 8mb of RAM was amazing). We had developed an application that worked perfectly, except on this one salesman's laptop - where it would only work occasionally. What I found was they had installed "Tiger Woods Golf" on their machine and it had a DLL with the exact same name as ours. If the salesman played the game before running our application, Windows said "Oh, I already have that DLL. No need to load it again." and our app would blow up.