THERE ARE MANY APPLICATIONS OF GAUSS THEOREM, THIS IS ONE OF THEM.

Answer:

Gauss’s Law can be used to solve complex electrostatic problems involving unique symmetries like cylindrical, spherical or planar symmetry. Also, there are some cases in which calculation of electric field is quite complex and involves tough integration. Gauss’s Law can be used to simplify evaluation of electric field in a simple way.

We apply Gauss’s Law in following way:

Choose a Gaussian surface, such that evaluation of electric field becomes easy

Make use of symmetry to make problems easier

Remember, it is not necessary that Gaussian surface to coincide with real surface that is, it can be inside or outside the Gaussian surface