The reason for doing this is simple: a warm boat beneath a cold deck and a cold inner layer of air will condense like crazy and it will be miserable inside...a dripping, nasty mildewy mess you can't air out.
It's no picnic in the winter, but it CAN be done in relative comfort with forethought and planning. I recommend "The Warm, Dry Boat" for more ideas..
Warm boat under cold deck... Will insulation on deck, under the wrap, take care of the condensation?
Warm, Dry Boat...is this a book or a thread on sailnet?