system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have to restart our PC after uninstalling an app?
gpt
Depends on the OS, most times, a restart is unnecessary, but in some cases it may be required.

###Windows:

On windows, applications are stored as files and registry keys. Registry keys are never cleaned properly, and an unnecessary large amount(talking a hundred uninstalled apps) may slow down your PC a bit (not by much though, it's just that windows loads every registry key on startup, even empty ones).

A restart is necessary if :
 
- The app has a daemon running, as executables that are currently run on your machine cannot be deleted
- the app has stored data in protected areas of your system (system32 for example)
- The app installed shared DLLs that are currently run by the system

###Unix:

Unix based operating systems use only files to store app data, and the executable files, restarts are only necessary if a daemon is running, even though linux kills pending processes to not interfere with the uninstall.

###And in general :

It's always a good idea since it may free up some RAM, and don't forget to defragment your HDD afterwards.