Long lines of text can be hard to read, so doing multiple columns breaks the lines up to something more optimal.

It was shown, by experiment, that optimal amount of text for print material is ~60 characters per column. This was calculated by the ratio between font size and leading which were picked by humans as most comfortable to process. Higher amount of characters per column interferes with brain's ability to scan through text easily, much like you need to break text into paragraphs for same reason.

Also when writing mathematical equations, there is a tendency to have a lot of free space on the sides. Using two columns not only to make text reading more comfortable, but it also allows to make a better use of the space on the page, by reducing the white space on the side of the equations.