A palm-leaf manuscript leaves very little space around its words. The leaf is narrow and precious, so the scribe fills it from one edge almost to the other. Paper changed that. Once it was cheap enough to leave some of it empty, readers found a use for the emptiness at once. They wrote in it.
Space to rest
The margin gives the eye a place to stop at the end of each line. Without it, the words run into the edge of the page, or the edge of the screen, and the eye has to find its way back to the next line with nothing to guide it. That is why a page with room around it feels easier, even when it holds exactly the same words.
Space to answer
Readers have always argued with books in the margin. A tick, a question mark, a word underlined twice, a short "no!" beside a sentence that annoyed them. A secondhand book with notes in it is two books: the one that was printed and the one a reader wrote back.
Space on a screen
On a phone the margin is small, and it should be, because the screen is narrow. On a computer the opposite trouble appears. A line of text that runs across a whole wide screen is very hard to follow. A good theme keeps the column to a comfortable width, somewhere between fifty and seventy-five letters to a line, and leaves the rest as margin. That room is also what makes reading slowly feel natural rather than like hard work.
Look at this page
On a computer, notice how far apart the two edges of this column are, and how much room is left on either side. The theme sets that width. The writers did not have to think about it.
Next time a page feels tiring, before you blame the words, look at the space around them.
This piece has two writers, so that you can see how Alaya Pro lists more than one.