There is a great story about Stephen Venables – fresh from his ascent of the Kangshung Face of Everest, being asked to take a Blue Peter presenter climbing on the Ben. At one point the guide who was in charge of safety asked Steve to tie in with an alpine butterfly, as this knot would take a three-way loading. Unfortunately, although Steve was one of the stars of Himalayan mountaineering, he wasn’t so hot on his knots, and so could only answer that he hadn’t a clue what the guide was talking about.
Luckily for me, I used to work in a shop full of cavers – who are probably the biggest users of this knot (the ‘caving knot’ doesn’t have the same romantic air to it does it), because it can be untied easily even when severely loaded – crucial when fixing ropes for SRT. It also has the advantage of being able to take a three-way loading, something the figure of eight doesn’t, which makes it the knot to use when tying on to the middle of a rope for glacier crossing and mountaineering (hence the Alpine tag). For me I find it useful for setting up top ropes, fixing lines and anywhere where ropes are going to be loaded heavily – saving you the frustration of trying to untie a figure of eight that has seemingly turn to stone.