Uses of Electronic Filters?
It is an extremely vast topic. Also, an extremely important one.
It's a whole field of study within electrical engineering.


  Why do we need certain range of frequencies while others are rejected ?


Depends on the specific situation.

For instance, if you're building a radio, a big part of the whole spectrum is occupied, so you need to select a narrow portion to be able to hear a single radio station.

In another context you may need to eliminate as much electronic noise as you can, so you can have a "clean" signal.

Or you may have a system that is on most of the time, but inevitably turns off every X seconds for a very short time, but you may just wanna know if it is on or off "on the long term".

These are just sparse examples intended to spark your curiosity. If you're interested, have fun taking your time to learn this wonderfully interesting area of technology.