Many themes do not provide a design for the view of links or other focusable fields when a keyboard user brings focus to that element. This makes it very difficult for visual users who are keyboard dependent to keep track of where they are.

A common pattern for this is to change the appearance of the element on keyboard focus; and this item adds an outline. Outline is rarely defined in most designs, so that ensures that the definition by this plug-in will almost always work and be visible; the same can’t be said for text decorations like underline, since that may be the default state for links, and therefore would not be a change indicator.

Further reading about keyboard focus.