I (we) use them two main ways:
First, I do static bar holds to train supportive grip for deadlift. Typically, this is simply holding the bar for around 15 secs or so at the top of the last set of deadlifts, unless this is too light so that the hold could be held too long, in which case more weight can be used from a racked bar position.
Also, I prescribe heavy bar walkouts. The purpose of this is not so much core training, which is the usual reason, but to simply get ready for holding heavier weights in squats. This is for a psychological/physiological purpose as well as often a lifter can fail simply due to the shock of the feeling of a heavier load on his shoulders. This kind of thing is most useful when we know that there will be heavier max loads than the lifter is accustomed too due to a reduction in intensity. It's sort of a introduction to what is to come.