In some ways. many guys, including me, love to have a little physical feedback. We don't have that convenient easy measure like you women do with the male erection. A clitoral erection is a bit hard to judge some times. Intravaginal infrared photoplethysmography really does the trick for measuring physical arousal, but it's mite bit of a turn off, unless she has a bit of a medical kink.
So I guess, over all, it's just reassuring to see/feel a physical response. Sense of pride and all that. .
'Course there are many things that can effect the production of lubrication. It's absence does not indicate failure to arouse, but it's presence is an indication of success. As men we'd like to feel like we did a good job. Even if it was, in actuality, caused by the sight of some hot girl across the room or the though of some hot toy in the sock drawer. ^_^
Just my thoughts.