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.
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