They are actually meant to keep you dry as you perspire (yes you still perspire in the cold!), and help regulate your body temperature.
This will prevent you from getting cold by transferring perspiration from your skin to the outer layer of the fabric.
A good base-layer keeps you dry, comfortable & warm.
To be effective, find a base-layer that is made of wicking materials – these are special fabrics that help draw perspiration away from your body, pushing the moisture out to your middle & outer layers.
These fabrics are made to provide better absorption of perspiration using a 3D structure as compared to other natural fabrics like cotton, which makes it easier for your perspiration to evaporate.
The quicker it wicks away your perspiration, the warmer you'll feel!