It's for situations when you are observing incoming traffic to evaluate opportunities to safely cross paths with it:

The notion behind front brake lights is deceptively simple. By signaling to oncoming traffic and vehicles approaching from the side, a front brake light provides an essential visual cue that a car is slowing down or preparing to stop. When the light is extinguished, it indicates that a stationary vehicle might initiate movement.

When I'm in a parking lot turning left onto a main street (US, drive on the right side of the road), and a car is coming down the street, [maybe] turning right into the same parking lot, I won't pull out into the street in front of them until I can see them decelerating. The risk that they aren't turning and would then T-bone me is too high.

Heck, even if the turn signal is on, I'll wait, unless I know I saw it previously off for that car (in case they're driving with their signal on, not intending to turn)