In the dry, if I stomp my foot down or press really hard, my foot would (I use "would" because I am not silly enough to press hard) get ripped behind me because of the high friction. So, in the dry, I have lightly put my foot down (controlling the force) to feel the friction felt on my boot. It is also a good way to appreciate, or test, different traction levels on different surfaces (concrete, old asphalt, newly repaved asphalt).
Similarly, in the rain, sometimes applying lots of pressure on the road will yield very little friction (my foot just slides with almost no resistance). Same goes for different surfaces, also in the rain. Believe it or not, on the DVP there are several different road surfaces with different levels of friction, even in the rain.
This kind of knowledge makes a difference to me.
As for your question about the correlation: friction factor is friction factor. The correlation may not be direct, but it is proportional (I would argue, linearly proportional). What I mean is: If I feel "half" the friction with my boot, I assume my tires have half the traction as well.
