An easy button for making webpages more readable when the text runs across the width of the window on larger displays.
First click, it increases the page margins. Second click, increases it more. A third click returns to the webpage default, for cases where the page doesn't place nice and we want to revert to the origin margins without reloading the page.
It's easier to read when the text falls within a more narrow space, such as the columns of a typical book or newspaper. The mind follows the text and takes it in more quickly.
Many sites don't consider that a lot of reading is still done on non-mobile screens, or that it's annoying to have to resize a window to comfortably read an article. This can be a frustrating user experience.