Even you don’t care about mobile devices, it still worths it made responsive
Because visitors might Split View the browser and editor.
You don’t have to be a nerdish tiling window manager to arrange your windows like this in less than 5 seconds:
And it will be nightmare if you have to horizontally scroll to read a full sentence.
MDN here is a good example; Stackoverflow is not. It does implemented a mobile view but requires finding this tricky link and to click and refresh the page.
Having considered layout under a narrow viewport does not only benefit iPhones and iPads but also those who try to make better use of their Hong Kong downtown displays.
A 15" RMBP has a screen corresponding to 1440×900 in resolution, whose width cutted in half goes to 720px. The figure shrinks to 640px of a 13" model. Now we’ve got two good starting points.