Unit Existence: UnitExists vs UnitGUID

[stako]

Having a look at the Blizzard_ArenaUI addon, there's a line to check for the existence of a unit:

if ( UnitGUID(self.unit) ) then --Use UnitGUID instead of UnitExists in case the unit is a remote update.

What does it mean by 'remote update' here and what's the difference between using UnitExists and UnitGUID when checking for the existence of a unit?