The generated HTML from /images/Structure.jpg (Data Flow) is:
<figure>
<img src="images/Structure.jpg" alt="Data Flow" />
<figcaption>Data Flow</figcaption>
</figure>
So, the figure caption doubles as the image’s alternative text, making it easier (and less repetitive) to add captioned images to a document quickly.
There was also some discussion earlier about making alt text optional in some scenarios (e.g. quickly adding a set of images to a forum post), so this might be a nicer solution for those cases (no empty square brackets). /images/Structure.jpg produces:
<figure>
<img src="images/Structure.jpg" alt="" />
</figure>