The phase transition from/to solid/liquid certainly requires extra heat input/output.  This is the process they take advantage of in FL citrus.  In that case, they're trying to prevent any plant tissues from dropping below 32 and freezing, as that will damage and/or kill the trees fairly quickly.  So they're using it to prevent the trees from freezing at all during a very mild frost spell (think 28 or 30 degrees overnight low).  They're effectively creating a tiny microclimate around each tree that is at or slightly above freezing.  And even if a little water freezes, that will happen from the outside in, so the water will freeze first and possibly spare the tree.

Certainly it would be a different principle in the case of spraying a fig tree with water when you know it is all going to freeze, thereby using up all the phase transition heat.  Once it is ice, the temperature will fluctuate with outside temperatures fairly quickly.  It may be effective at preventing tissue damage in non-citrus trees though.  I'd be interested to hear the theory behind it.  Is it to prevent dessication, like George suggests?

Well old man winter is back. I spent today moving everybody back into the garage. Tonight and Sunday night it is suppose to be below 32F and almost all my trees have broken dormancy. Frost is projected in z9b. Anybody else have a sore back.