Because many users of screen-readers use links to navigate the page, providing links with no text (or with images that have empty "alt" attributes and no other readable text) hinders these users.

Example

Wrong

<a href="home.png"></a>

<a href="home.png"><img src="home.png" alt="">/</a>

Right

<a href="home.png">Return Home</a>

<a href="home.png"><img src="home.png" alt="Return Home">/</a>