There are no legitimate uses of this technique. It only applies if you lack the consent of the target. If have that, there are many easier and better ways to monitor them.
A personal home security system could make better use of this instead of motion sensors in each room.
As mentioned elsewhere, detecting earthquake victims maybe, a terrorist or hostage situation where the targets definitely aren't giving consent, military situations and covert surveillance for law enforcement and counter espionage all spring to mind as legitimate and useful applications.
SWAT team dealing with a hostage situation in a closed room
It can also be used to monitor a baby's respiration