As is always the case with such things, they are not selling the image but the convenience.
The one linked in their example is on the first page of Getty search results for "old telephone sketch", along with dozens of other similar options. Being able to pick one I like in 3 clicks is easily worth the $500 when compared to having to go to the library (or Google Books or wherever else), digging up old books that may or may not match the subject, finding appropriate illustrations, hi-res scanning, editing in Gimp...