Since the Earth rotates on its axis, each point on the Earth is traveling in a circular path about that axis. Each point takes 24 hours to complete its path.
We can see from the below picture that as we descend in latitude from the north pole, the circumference each point has to travel gets greater until we reach a maximum at the equator.
Since a point at the equator has to travel the greatest distance in 24 hours, it must be traveling the greatest velocity.
At the equator, a point on the surface is traveling east at 1037.56 mph (1669.8 km/hr). A rocket sitting on the launchpad would also have that speed. So, if the goal is to accelerate the rocket to orbital speed, it saves fuel if we start at a higher speed.
More questions on Space Exploration:
- Did the Columbia crew have any chance of surviving?
- Will SpaceX put a human on Mars before NASA?
- If all human life on Earth were extinguished, how possible would it be for any remaining humans in space/orbit to return to the Earth (alive)?