Besides keyloggers, another reason people want this is because law enforcement has USB keepalive devices that will simulate mouse movement/keypresses to keep your computer from going to sleep.

They do this to make sure your computer stays on and your RAM doesn't get powered off, which will allow them to read any decrypted data in memory whether or not your data is encrypted on disk.

When they raid you, they come with massive UPS devices that they plug your computers into to give them as long a window as possible to get your data.