There are all sorts of reasons why you might need a whole house dehumidifier, regardless it it's winter or summer.
Dehumidifiers can be used to control condensation on windows, walls and ceilings that would otherwise make its way down to the wooden frame of the house and cause rot.
Traditional ways to control condensation include a technique which is known in Germany as "Stoffluften". That's the act of throwing all the windows in the house open in the middle of the winter to vent all the damp air out of the house. Cold air rushes in, the windows are closed again, and the heating system warms the air back up usually to a humidity level that was lower than before.
Dry air feels cooler and allows you to turn the temperature down
Preventing dampness is a huge deal in crawlspaces on some parts of the world. Dampness in bathrooms is another
Dehumidifiers can keep relative humidity from exceeding 50% which is the baseline for mould growth. That's the point at which dampness starts to deposit on interior walls, ceilings, floors and windows. If that dampness remains for any length of time, mould spores and start to grow in it. The longer the walls remain damp, the deeper the mould penetrates until the wall material, the wood framing beyond all start to rot.
When air is less than 30% relative humidity it can be uncomfortable, your feet generate static electricity. Your nose bleeds frequently
It's cheaper to dehumidify air than to heat it
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