On most devices it allows for an image to expand to the full size of the display screen while maintaining it's aspect ratio. By setting the image size(WxH) and the viewbox(WxH) to the same numbers, you can then use other functions like height/width at 100% for max screen usage, or 50% for half the screen; preserveAspectRatio="xMidYMid meet" will center the image vertically and horizontally.
I like using it for images that take up a whole page, like maps and covers. If I want an inset image then I'll use a <div class="yadda"><img alt="" src="../Images/example.jpg" /></div> option. That way I can style it as desired for size, positioning, text wrapping, etc.