They are primarily good for landing jumps backwards, rounded tails also wiggle through bumps better.
So Mr. HowcanIbepostingthissinceIamamaggottrollfromTGR,
1) They look cool like a snowboard so they tend to appeal to younger skiers. Also they tend to have ghetto motifs with guns and corpses, which may be appropriate given number three below.
2) If they are suitable for pipe and park, they tend to have a progressive flex pattern which allows the skier to land backward ariel manouvers (fishy tricks, flounder).
3) As others have noted, they beat the hell out of unidirectional skis for skiing "switch" (backwards). Why younger skiers would want to ski backwards fast
beats the hell out of me, since they seem to have no problem hitting you when skiing forwards fast.
4) Really fat twin tips work much better for pond skimming championships. Some have taken this to the nth degree and produced twin tips that look like water skis. See eg. the K2 Pontoon, which will not carve a turn no matter how much you angulate.
5) Larger fatter twin tips with an aggressive turning radius have the advantage of skiing 10-15 cms shorter on the groomers than they do in powder. This means you get a ski that will turn reasonably quickly on groomed while still having big ski float on powder.