most of you touched base on the main ones. other reasons:
if you wash it by hand, youll just spread the salt and dirt around. for you to actually wash the car well, youll have to replace your water and wash mitten constantly. one way to do is a two bucket system with grit guards.
the best things to do in the winter time is just power spray the car often so theres not huge build up. if you do end up washing it by hand, as soon as you get on the main street, your fenders will be covered in salt deposits again.
let the rain do the rest. now that its getting warmer, hose down the car, the rain and driving in the rain, will clean out your fenders for you. i recommend cleaning out your door jams by hand. if you take a look now, youll find sand and salt in your door jams. leaving the build up in there is what will cause you problems.
or a quick search can help: