Why do I need Air Conditioning in the Winter?

The quick answer is this: defrost.  To explain further, the A/C system doesn't just cool the air, it 'conditions' it.  Whether you want cold or warm conditions is your choice.  The A/C system is also a dehumidifier, removing moisture from the air.  Most defrosters run with the A/C system these days.  Also, because most engines have a single serpetine belt driving all the accessories, a locked up compressor can cause further problems, including a broken drive belt, resulting in a stranded motorist.  If you are not going to replace your locked up compressor, do not turn on your A/C or defrost.