I started using liners to prevent blisters with heavy mountaineering boots. Now I use them for extra warmth mostly. Like any good base layer it transports moisture away from your feet, dryer feet are warmer. Silk, wool, synthetic all work to different degrees. They also prevent hot spots that can turn into a blister. If I am using liners I can usually wear the outer sock longer so need less pairs of heavy socks.

The liners especially synthetic can easily be hand washed and dry in 30 minutes where heavy wool socks can take a day or more to dry.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli