Why does just hitting CTRL-ALT-DEL sometimes unfreeze programs?

Task manager is a what I like to call a mission critical application. It as you said can be used to close non responsive applications. But once it opens it is added to the list of running applications that the CPU needs to switch between. When you use your computer the CPU is constantly Switching between programs or sharing the load between multiple cores ( if your processor is a multiple core processor). But Task manager has higher priority over all user applications and by wedging it front of the frozen application it can clear out the lock up. This includes the desktop if it locks up too because it is also a application.