Why we have to know refractive index and can use this in normal life?
Well, of course if you happen to be a spear fisherman who spears fish from above the water, you need to correct for refraction at the surface.
More commonly, if you wear glasses or use binoculars or a telescope or a microscope, lenses work by the diffraction of light at the surface and you figure out how the lens works by following refraction of rays through the center of the lens and coming in parallel. Yhose of us who work on the ocean spend a lot of time working with refraction of water waves, around islands and breakwaters and such. People from inland often wonder why waves tend to come in with crests parallel to a beach when they are at some angle to that offshore. The answer is refraction. The wave slows down as it comes into shallow water and the part of the crest farther out is still moving faster so catches up, turning the crest toward the beach.