If you don't have your ultimate up you can use you predator when you gank to get that movement speed and reach the enemy faster, then you can E him for slow. If they had pushed your ally under the tower or he just pushed him hard this is the perfect situation for your predator. The best moment for this keystone is when you have your ultimate, that will give you even more speed as you know and this will make you catch enemies faster. In some situations you can escape with this keystone.
Sorry i can't explain it better just try it to see how powerful it is on Renar.