A launch near the equator towards the east direction will get an initial boost equal to the velocity of Earth surface.
Here's the answer to the question asked by A. Karthika from Chennai:
Before we discover the reason, let us know about the Earth’s rotation. When seen from the North Pole, the Earth rotates anti-clock wise. It means in general term, from west to east as we see the sun rising in the east and setting in the west.
The surface velocity of rotation varies from point to point on the Earth. It is about 1600 km per hour or about 460 meters in a second near the equator.
The velocity gradually reduces as we move to the poles and it is practically zero there. A satellite launched from the sites near the equator towards the east direction will get an initial boost equal to the velocity of Earth surface. This is similar to an athlete circling round and round before throwing a discus or a shot put.
The initial boost helps in cutting down the cost of rockets used to launch the satellites.
This is the major reason for launching satellites in the east ward direction. But this benefit can be taken only for such satellites which are placed in geo-stationery orbit or which circle the Earth parallel to the equator. Such satellites are usually communication satellites or satellites used for scientific research such as ISS.
There are other satellites which are placed in polar orbits moving across the equator in north south direction and used mainly for mapping or some times for spying. Such satellites are generally launched in south ward or north ward direction and therefore cannot take advantage of the Earth’s rotation.
Another characteristic of launching satellites is that the launching stations are generally located near eastern coast line so that, just in case of failure of the launch, the satellite does not fall on built-up hinterland.
