I believe they can be installed so that it automatically goes off when the lifejacket inflates, but this requires some integration of the lifejacket and beacon.
The ones I have seen have been manually activated.
While the life tag will put a GPS
position where the MOB occurred the AIS beacon will give a continual update of the position of the drifting person and display this on the chartplotter
I have never had a real MOB, but those I have heard about often occur with some sort of problem (like a spinnaker
tangled around the forestay) in these sort of situations it may take some time to return to the person in the water especially as the crew will now be short handed and possibly less experienced (if the skipper
is lost). Knowing the position of MOB seems a major advantage.