When people lay out desktop sites, 960px is the smallest common minimum design width. Many of these sites are statically laid out, and if rendered with the width of 949px would require horizontal scrolling.
Yes, if these sites were not statically laid out, and instead gracefully handled any size they happened to be rendered at, providing a viewport width of 949px would not be a problem.
On many sites, this trigger the appearance of the dreaded hamburger menu.