A few posters have suggested that if, indeed the aircraft was hijacked/stolen, the hijackers/thieves would likely want to communicate with a ground crew somewhere, and for this they would need the aircraft's SATCOM capability.
I beg to differ. A simple satellite phone would do nicely. They are small enough to easily be taken on as hand luggage.
Coincidentally, a satellite phone call could well be handled through Immarsat.
Immarsat haven't said precisely what data they handled. Read into it what you will.