As an aircraft dispatcher, I had an ACARS window open at all times on my work desktop. Same principle as sending and receiving texts. I would use it to advise the crew of changing weather, new numbers in the event of a configuration or operational change( losing an AC pack requiring max altitude of 250, meaning new fuel numbers), or junioring a crew(they hate that). There are other versions that connect with maintenance control, providing data..
The maintenance messages that are automatically sent advising of abnormalities would be the ones to be expected in a dire emergency. Air Malaysia says it received none, and that is puzzling.
In the AF447 crash, ACARS messages pretty much laid out the initial problem, the recovery of the black boxes confirmed it, and showed poor airmanship during attempted recovery.