Regarding the usage of media query

Question

i was reading a article on Web Application to Display Size Compatibility from this url http://www.c-sharpcorner.com/UploadFile/b19d5a/making-Asp-Net-web-application-to-display-size-compatibility/

 

i saw the person write first css for few element like #nav,#content,#extras etc but again he defined more or less same css inside @media screen like (max-width:380px) { } for those same element #nav,#content,#extras etc. it seems that he is duplicating css for same element #nav,#content,#extras etc..........why ? is it require when some one write media query then he/she need to define css twice for the same element like one general css and again write css inside @media screen() . please guide in details. thanks