Sneak can move Clingers, or any of your many 0-cost units to that useful place. 
Cave Goblins can have trouble moving the horde, with so many units on-board and only 3 movement actions and Sneak helps with that.
This event is what also makes it okay to just drop a lot of Cave Goblins in random places. Sneak can suddenly place those units where you need. 
It's very important to note Sneak is a Move Phase event, which means it can be used at any point during that phase, before or after any unit moves, so the possibilities are endless. It’s not the most powerful effect, but it really helps Cave Goblins to position their many units.