If the only force acting on an object is gravity, we automatically know some things about the kinematic variables and can use this info to predict motion. For example, the acceleration in the vertical direction will always be 9.81 m/s/s down, because gravity accelerates everything on Earth at this rate if there is nothing pushing up on the object. If we "drop" an object, that tells us that the initial velocity is 0 m/s, because it is not moving up or down to start off. If we want to know about the maximum height that an object reaches, we know that the final velocity of the object will be 0 m/s, because an object will rise until slowing to a stop before falling back down in the other direction.
