If it is not written in bug report that description is wrong skill should work as described in tool tip imho.
Snare affects movement speed only (which is better imho), immobilize makes target simply unable to move- in some games it was actually worse because affected mob switched immediately to ranged attack instead of chasing it's target.
If I recall well all classes who had no sleep type control effect received one. It usually has cast time of 5sec at rank 1 and information that it will break if affected target gets a hit when under 90% of hp.
To summarize if You use immobilization type cc on mob, AI script might switch to less powerful ranged attack and also ignore aggro generated by tank and select nearest target. The result is wipe of melee class in range