reflector antenna used

why is parabolic antenna with horn feed used??

What you want to do an antenna is to transmit as much of your power as possible in the required direction, ideally like a searchlight beam but with parallel sides. This is what a parabolic dish does. The feed horn has to be at the focus point of the dish. Likewise a receiver only has to pick up RF energy from one direction, so it uses a dish as well.
Frank

A parabolic reflector is used as a part of an "optical" antenna design.
a) The prime-focus design utilizes typically a horn (or a dipole, etc.) located in the main focus. The horn in the focus and its mounting bars cause shadowing and affect the radiation pattern of the complete antenna. If a low-noise receiver or a high-power transmitter is located next to the horn, its enclosure increases the shadowing and its weight is a problem. Then we have other "optical" designs to use:
b) Cassegrain system has the secondary (hyperbolic) reflector, smaller than the main dish, located before the primary focus. The secondary focus is located close to the main dish vertex. While there is still shadowing due to the secondary reflector, the heavy and bulky equipment is located suitably behind the main dish.
c) Gregory system has the secondary (elliptic) reflector, smaller than the main dish, located behind the main focus. The other points are located like in b), the shadowing is lower.
d) offset antennas of all above types are used in satellite communication and radio astronomy, to remove the shadowing and to reduce filling the main dish with rain water and snow.