Just like in skiing where a lot of people say speed hides a lot of crap . In mountain biking its the same momentum is key for any one who wants to start riding more technical trails, its just as important to learn how to balance where no momentum is present, because there will be times where you will lose all of it.
Track standing will help you most greatly in these areas.
Slow speed technical trails
elevated trail surfaces Ie bridges and logs and north shore style features
setting up for wheelie drops
and lastly it can help instill confidence in riders new to clipless effectively buying confidence and time prior to just toppling over.
Tracking standing is one of the most fundamental skills on a MTB or any bike for the matter. Its the key foundation to all other balance being done on a bike. there are several ways to learn it. Fundamentals in a track stand are. 1. Looking up, 2. using resistance at first to move against (front brake or slight uphill) 3. always moving.
The basics of a trackstand is that you always moving against something at first until your balance get good enough that doing it on flats and downhill is possible. Balance is movement and although track stands appears to be stationary they are anything but. You are constantly moving and correcting even when there appear to be no movements. Learn first by leading with your strong foot and ratcheting your pedals as your work against a hill or against your front brake or both. You goal at first is just to move slow, and then move slower until eventually you just are rocking back and forth against your brakes or the hill. If you lose balance DO NOT put a foot down but instead learn to pedal out to regain balance.
When to learn how to track stand?
I normally do it when I am waiting for other people to catch up, but quite honestly any free time you have on a bike a couple minutes a day learning this will eventually lead to mastery of the skill. If you want to get better at riding off road some of your time should be spent working on skills and not just trail riding.