why radars use in military?

Applications of radars in military:

1. Detection and ranging of enemy targets even at night.
2. Directing guided missiles.
3. searching for submarines.
4. Early warning  regarding approaching aircraft or ships.
5. Aiming guns at aircraft and ships.

The reradiated energy on being received back at the radar station gives information about the location of the target. the location of the target include range, angle and velocity parameters. the range is the distance of the target from radar station. the angle could be azimuth or elevation angle for static targets and velocity for moving targets.
for satisfactory location of target,the received power(echo power) must be appreciable. accordingly, the amount of power(energy)required to be radiated by the radar transmitter must be tremendous,typically few kW to MW.such high power at high frequencies can be generated using magnetrons.