Pressure switch for crawlspace ventilation?

So i’ve seen these a couple of times in 2000’s era homes with crawlspaces. The pressure switch is installed in the ducting.
ducting goes from exterior of the home, into the crawlspace.
exhaust fan is installed in-line.
Obviously for ventilating the crawlspace, which can help with mold/moisture and radon levels.
But I’m afraid i don’t understand the application of the pressure switch, and what function it is serving…
here is a link to a typical pressure switch.

From your link:

Model AFS-222 is a general purpose proving switch designed for HVAC and Energy Management applications. It can be used to sense positive, negative, or differential air pressure. The plated housing contains a diaphragm, a calibration spring and a snap-acting SPDT switch. The sample connections located on each side of the silicone diaphragm accept ¼” OD metallic tubing via the integral compression ferrule and nut. An enclosure cover guards against accidental contact with the live switch terminal screws and the set point adjusting screw. The enclosure cover accepts a ½” conduit connection.

Yes, but what it the purpose of the pressure switch. I understand installing a pressure switch to activate a lift fan to boost flow to a supply register that is not getting enough air flow.
But can anyone explain why one would be installed in an exhaust fan in a crawlspace…

Possibly, to let out moist air in the those months when needed.