Why pass a query parameter to an image file (/img/button.png?123456789) from inside a HTML page?

I saw this in a HTML template for a web app:

<img alt="mybutton" src="/img/button.png?123456789" />

What I didn't understand was why the ?123456789 would be passed to the image file? (in the开发者_如何学Python actual app, the number that was actually passed seemed to be customized to the user's session or unique id perhaps)

It forces the browser to skip its cache, and retrieve a fresh copy from the server.