After reading through the tweets, it seems that plane is very special and contains launch codes for nukes. If the president would be harmed or in general the security at the white house unable to operate, this plane takes over. A precaution, it seems, in case somebody wants to attack the USA. But as I understood it, starting the plane is more of a signal for defense readiness conditions (DEFCON) and helps signaling other planes and stations to be aware. It's pretty interesting IMHO that the plane started before the president announced it on twitter or anything, giving a solid hint that something is very off in DC.