Cold air has less of an ability to hold moisture, the colder the air the less moisture it can physically hold. So 100% relative humidity at 20°F is much less moisture than 100% relative humidity at 80°F
When you heat cold winter air to a comfortable room temperature you are increasing it's ability to hold moisture without adding any moisture. This has the effect of decreasing the relative humidity. The bigger the change in temperature the more you decrease humidity by heating it.
Winters are cold enough in some places that indoor relative humidity can easily get so low it effects humans negatively - dry cracked skin, dry cracked mucous membranes, sore throat, static buildup. Add to that low humidity aids in the spreading of viruses.
When you heat cold winter air to a comfortable room temperature you are increasing it's ability to hold moisture without adding any moisture. This has the effect of decreasing the relative humidity. The bigger the change in temperature the more you decrease humidity by heating it.
Winters are cold enough in some places that indoor relative humidity can easily get so low it effects humans negatively - dry cracked skin, dry cracked mucous membranes, sore throat, static buildup. Add to that low humidity aids in the spreading of viruses.