In the example on the left, a visually-impaired user might not be able to see the arrow (i.e., it's just a graphic, like any other picture), so it might not be clear how to get to the next page, etc. This consideration also applies to the popular use of graphics, rather than text, to navigate to other important things like "buy now", "shopping cart", "checkout", "home", "contact us", "warranty info", etc.
In the center example, we've put the words "NEXT PAGE" in black text; obviously, this is the most conspicuous means to confirm that pressing this button will take the user to the next page (of course, we recommend turning the "NEXT PAGE" into a link, which also takes the user to the next page).
In the example on the right, the color of the font for the "NEXT PAGE" link is in white; because the background is also white, most users will never notice it. You can see it if you click and drag your mouse pointer over it. This is a helpful strategy, for example, if your image has the words "next page" in it, but they do not appear as text; thus, someone with only limited vision might be able to see the text on the image, so some web designers might be reluctant to have the term visibly repeated on the same page; this arrangement, whereby the text is visible to text readers, but invisible to almost everyone else, solves this problem.