what is the usage of HTML5 figure with img

HTML5 figure is used whenever you use img, video tag with caption (figcaption) on it

If you have a number of related images (or other content) with caption text, you can use nested figure elements to associate both a group caption and an individual caption to each instance using the figcaption element.

<figure role="group">
 <img src="castle-etching.jpg" alt="The castle has one tower, and a tall wall around it.">
 <figcaption>Charcoal on wood. Anonymous, circa 1423.</figcaption>
</figure>

Here are some examples

Images are not technically inserted into an HTML page, images are linked to HTML pages. The <img> tag creates a holding space for the referenced image.

While the content of the <figure> element is related to the main flow, its position is independent of the main flow, and if removed it should not affect the flow of the document

From http://dev.w3.org/

"The img element represents an image," while "[t]he figure element represents a unit of content, optionally with a caption, that is self-contained, that is typically referenced as a single unit from the main flow of the document, and that can be moved away from the main flow of the document without affecting the document’s meaning."

The tag provides a container for content that is equivalent to a figure or diagram in a book. It can be used to group a caption with one or more images, a block of code or other content.

In other words, enables us to work more freely, realistically with images, diagrams by enabling to group them & caption one or images with a single caption.

Source: http://www.html-5.com/tags/figure-tag/index.html