It is a trivial setting - at least when using an Apache server.

The problem is that lots of people like to steal bandwidth from other sites by inlining other peoples photos on their own sites. So they get the server and network load, but no visitors seeing any sponsor links or the sites own articles.

This type of bandwidth stealing is a huge problem for image-oriented sites where a majority of the bandwidth are images - and a large percentage of the bandwidth is for inlining the images on other peoples web sites, blogs or similar. If the server owner have had to buy a more expensive hosting alternative (i.e. more bandwidth) and then finds out the amount of bandwidth for this kind of linking, it's quite easy to understand why the site owner would add this simple test to the web server configuration.