The chief reason is for picture assets. Before projects maintained all the radiant components of bleeding edge CSS (drop shadows, balanced corners, slants, and that is just the tip of the icy mass) it was outstandingly difficult to have cross-program effects without the usage of pictures. Fashioners would make shadows and balanced corners as pictures, and after that cleverness coding traps were used to put the imagery on the page. These preferences ought to be seen in any case, so making them meanwhile as the high dedication mockup truly saved time.
Other than (and possibly more fundamentally) the web used to simply be open on desktop programs and wasn’t for the most part present on phones and tablets in the way it is today