It is all about escape velocity, and for equatorial orbits and trajectories, going with the rotation of the earth does help. A lot. To the point that many of the standard rockets, it'll boost your launchable mass by 20%. That is why the EU launches from French Guiana, Russia rents back its Baikonur Cosmodrome from Kazakhstan, and Boeing et al. have a mobile oil rig looking thing that'll take your rockets down to the equatorial Pacific to launch it (Sea Launch)
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