Why it is important for an electrical engineer to know civil engineering drawing ?

Mrinmoy, I am assuming your reference to civil engineering drawing skills relates to understanding profiles and elevations of utilities.  Given the constraints of electrical conduit including conduit radius bend requirements, it is important the electrical engineer can understand all of the utilities that fall within the electrical pathway.  It is the responsibility of the entire design team (mechanical, electrical, and civil engineers along with landscape designers) to confirm that all of the proposed utilities can fit within the site. Someone who sees the big picture and helps solve the project challenges is more valuable to the project team than an engineer who can only address the calculations and theoretical elements.