Most books and magazine articles are printed with full justification, namely with both left and right justification, even though it is generally accepted that the most readable of the justification options is justification on the left only, with the right margin ragged.
(When I looked for some sources for this, I couldn’t find a single one that says otherwise. The accepted view is that we find it easier to keep our place in a text when there is a rag on the right, as compared with when the lines of type all end at the same distance across the page, and that consequently text is more readable for us in the first format than in the second.)
So what are the advantages of using full justification?
The main argument I always hear (I work with scientists, and they say it a lot), is that it looks better at first glance*. For a lot of people, the ragged edge looks disorderly and chaotic. On a first superficial look, having two straight margins to your text seems very neat and ordered.
Also, again especially in science, there is inertia. ‘This has always been the way we have done things’ is a ridiculously strong force, and just about every big publication in science justifies their body texts.
* This apparently subjective qualification is—thank you curious in the comments—well-explained with Gestalt psychology. The justified ‘block’ of text is a simpler shape and thus looks like it’s easier to make sense of than a way more complex-looking ragged-edge piece of text.