Well at first it doesn't make much sense to have that skill BUT there are applications.
For example if you want to use uruk captains to help you throw down a warchief but then you don't have much use for them, because outside of that fight they are just gonna wonder the map. They aren't gonna be at your side all the time. So you can brand them, kill warchief and then dispatch them all.
I can think of rune farming maybe. I've seen many people do it. You brand an Uruk or Warchief, then you level up your branded uruks and then BAM DISPATCH you have yourself some runes.
These are applications I've seen and could think of for dispatch. Because I don't think Monolith would just let that skill go through if it didn't make sense in the game.
I just want to add that the skill only kills uruks near you. If you have a branded Warchief on the other side of the map it won't kill him!
There seems to be missions and achievements that Require Dispatch or Dispatch is useful in.
one example the No Power in Numbers Achievement/Trophy where the dispatch is really useful to complete it.
And the Sword Legend Missions in the mission: Power of the One ->
This mission has Special Requirements: Combat Drain and Dispatch Abilities Unlocked!
and Main Objective: Make 10 Uruks kill each other by branding them, Eliminate all Branded Uruks.
An image as reference:
So as you can see the dispatch is useful in bringing all of the down with style and quickly.