Why they matter
Do them well, and skimo transitions will be an insignificant portion of your race. Do them poorly, and you'll lose to that guy you can't stand.
At the top and bottom of each leg of a skimo race, racers have to adjust their equipment. The adjustments are called transitions because the changes allow racers to change between skinning, skiing, and bootpacking.
Mastering each combination is essential in order to make transitions an almost-irrelevant part of a race. If you screw them up, they add unnecessary time to your race. Your race result will haunt you with what could have been if you had practiced.
This is especially true in the sprint format. A skimo sprint course takes between four and six minutes to complete. In such a short race, a botched transition can be the difference between a decent result and the back of the pack.
Skimo transitions are an essential skill in skimo racing. Done well, the time they take is insignificant when compared to the length of a typical skimo race. Done badly, transitions can add minutes to your race time and force you to finish well behind other racers of similar fitness. That sucks.
But anyone can have fast transitions. All it takes is practice.