Say, for instance, you have a clear picture in your head and can't think of any words to search and find something associated with that picture. Like, an image of a painting, but you don't know the artist, the time period, or anything else about it. Just draw a rough sketch of it, hit "find," and this image-searcher would display a bunch of possibilities. Once you find the right one, it can then tell you the artist and the title, so on.
Another example is drawing a symbol you've seen, and finding out its name and meaning, if it's a known symbol.
Crossposted from http://fayanora.dreamwidth.org