Tests show that a column of text is easiest to read if it is about 60 characters wide.

< - - - - - - - This is sixty characters - - - - - - - - - >

Try it and see. Just resize your browser window and watch the text flow. Make the window too wide and your eye loses track flying back along the long lines. Make the window too narrow and your eye tires from flying back to the start of the line too often.

Some early word processors had their monitors turned through 90° to get a portrait screen instead of a landscape screen. That is not such a good idea. Often you want two documents side by side, and a big modern landscape screen has room for two tall documents side by side. If the browser is displaying the webpage for reading it should use two columns. If the browser is displaying it for reference, for example, while a response is prepared, the browser window will only be across half the screen and single column would be best.