Ready lets you:
1> Do part of your action before hand, such as getting into a better position, healing an ally, etc
2> React to someone's turn in the middle of their turn
3> Apply Sneak Attack damage twice. Move into place, Minor for Low Slash + sneak attack, Standard for Readying whatever at-will for whenever whoever goes next does whatever they tell you they will do (make sure it's an ally and the trigger can be something as easy as "they attack" or "they move".) Since you are attacking on someone else's turn... sneak attack!