Great question — answering as the UphillTech team:
Hiking blisters are from friction. When things shift, your sock is more likely to stay with your boot than your foot as the two fabrics (or leather and fabric) will catch each other. This leaves the sock moving against your foot, which causes friction. When you wear two socks, specifically a smooth liner and a wool hiking sock, the outer sock moves against the inner sock instead of against your foot and less friction is created. Less friction = fewer blisters. For the same reason, two pairs of wool won't work as well as what you currently do because the wool is likely to bind and behave like one sock instead of two
Full answer by Stereotypical: https://outdoors.stackexchange.com/a/1014 (CC BY-SA 3.0).
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