One advantage in print; pages are often not entirely opaque. So on page 3, you see a bit of the text of page 2 shine through. If page 2 has a jagged right margi

One advantage in print; pages are often not entirely opaque. So on page 3, you see a bit of the text of page 2 shine through. If page 2 has a jagged right margin, this also makes the left margin of page 3 look uneven. Meanwhile, the right margin of page 3 looks has a "shadow" that goes all the way to the right because the left margin of page 2 pokes out underneath it - your eye might want to glide to the end of the block, but the sentence stops earlier. It looks better when it's aligned, which is also why you can specify a left- and right margin that alternates on odd and even pages. This is obviously only a concern for books and papers, and not so much for e.g. glossy magazines.