Imagine a person posting a diagram within a Stack Exchange question. Typically they wind up with markdown like this:
### ![enter image description here] : http://i.stack.imgur.com/TvhYx.jpg
The problem is that many images are too small when scaled to a normal screen, and we need to click them to make the image large enough to read. Usually this means I replace the aforementioned markdown with an
<img> wrapped in
<a href="http://i.stack.imgur.com/TvhYx.jpg"> <img src="http://i.stack.imgur.com/TvhYx.jpg"> </a>
That html makes the image clickable:
That's too much work. This is a rather common use-case; markdown images should be rendered as clickable by default.