Did you ever hit something with just 1 front tire, and your handlebars want to remove themselves from your grasp?? Well a steering stabilizer absorbs some of that blow, and helps to keep the bars in your hands.. There are countless stabilizers on the market, and I believe most of them are good, although some are rebuildable, and some are not. As far as for mx I would say they are good in any situation where your bars get jolted from some sudden impact whether it be a stump, a racer next to you, a bad landing etc.
Although Im sure there are other opinions I would say they are nice in any conditions.