The Reason

Primarily, silliness. The author needs a break, big-time.
Secondarily, the exercise enforces clarity and cohesiveness of construction, because to work at all even silliness needs to be coded correctly.
Third ("tertiarily" for you grammar-nazis out there--and you know who you are--), my family has for some time now lobbied for pictures of our growing menagerie. Using this site saves postage and printing costs, and relieves the minds of our loved ones who worry that we've become completely crazy cat owners. Saved by technology: if you're able to upload it to the web, you are officially merely eccentric.
Fourth (we'll just drop any pretense to fancy adverbs right here, ok?), it saves time on the assignment to pick a subject where there are already a lot of images to work with...