We know that earth revolves from west to east about its polar axis. Therefore, all the particles on the earth have velocity from the west to east. This velocity is maximum in the equatorial line, as V = Rea, where R is the radius of earth and (0 is the angular velocity of revolution of earth about its polar axis.
When a rocket is launched from west to east in equatorial plane, the maximum linear velocity is added to the launching velocity of the rocket, due to which launching becomes easier.