Having never seen such a pump I have absolutely NO IDEA. Sorry, I defer to others.
looks like it bumps up the idle, I bet if you could see under that top cover the wire goes into you'd figure out what it does.
What you circled is probably just a plug holder.
The wire probably goes up to a throttle position sensor.
Complete guess, I have no experience with this pump.
Might be handy for an electronic VNT-15 turbo controller.
UPDATE
I found out more. When you disconnect this plug on pump egr solenoid stops working and egr shut closed on idle. So this is some part of egr sistem.
My vote is throttle position sensor too. My guess is that the EGR behavior you noticed was in response to the system losing contact with the TPS when you unplugged it.
I thought that my 1991 MK2 pump was unnecessarily complex.
That one makes mine look like a Tinker toy!!