The main reason is for picture assets. Before projects maintained all the wonderful components of front line CSS (drop shadows, balanced corners, slants, and that is just the tip of the chunk of ice) it was astoundingly difficult to have cross-program effects without the use of pictures. Fashioners would make shadows and balanced corners as pictures, and afterward crafty coding traps were used to put the imagery on the page. These preferences ought to be seen notwithstanding, so making them meanwhile as the high dedication mockup truly saved time. Other than (and perhaps more basically) the web used to simply be available on desktop programs and wasn’t for the most part present on phones and tablets in the way it is today.