Why is reference point require to calculate relative velocity​

Others have mentioned that you need a reference frame to define your position and velocity. Depending on how you define a reference frame you can get very different results for the velocity of an object. For example, if you are throwing a baseball in your back yard you will probably refer to the ball’s speed relative to yourself standing stationary on the ground. This could give you velocities of ~50 miles per hour (22 m/s) for an average person. However, if you decided to define your reference frame relative to the center of the earth. Since the earth is rotating once every 24 hours and it’s circumference is ~25,000 miles (40,000 km) the ball will be traveling with a velocity of ~ 1000 mph (460 m/s). Hope this helped!