A suckdown winch is used to compress the front suspension when needed. When doing really steep climbs it can help keep the front end from unloading causing front to lift and feel sketchy. You can also use it to help with body lean sometimes. Another neat trick is that when you get turtle'd out on the belly pan in the snow you can suck the front end down which will help compress the snow under the car by adding sprung basically. Then let up on the winch and you can usually drive out backwards.
It's just another tool in the toolbox.
Thanks, thats what I was assuming but I couldn't think of any reason why you'd want to do it other than to get in a garage or something. All makes sense now though.