In addition to the reasons above (911-ish & transponder out), it is also an indication that somebody on the ground is aware and cares if communications are cut off and passengers do not know what is going on. They may not be able to do anything and passengers may realize that, but at least you know the ground is aware and you aren't alone.
As far as an on board nut-job being aware of any possible "trouble" he/she may get into, I think they usually don't care if they are 911 fodder.
When I heard that a second plane hit on 911, on the way to work, I knew life (mostly @ airports) had changed right then.
EDIT: I don't understnd the essence of the proposed edit on this comment. "fix formatting". I'm new here.