GLSL. Can someone explain why gl_FragCoord.xy / screenSize is performed and for what reason?
First, gl_FragCoord.xy are screen space coordinates of current pixel based on viewport size. So if viewport size is width=5, height=4 then each fragment contains:

Why are uvs needed? For example I rendered geometry to screen quad and then I need to apply some postprocessing on this quad in another rendering pass. To sample from that quad I need texture coordinates in range [0, 1]. So to calculate them I perform division gl_FragCoord.xy / viewPortSize.