Whenever you need the URL of one of your own actions, say, to build a button in your web app that links to that action, you can just write it like this in your HTML pattern:
<a href="/about/contact">Contact</a>
But doing so, if you ever change your action parameters or path components, you'll be forced to change your links manually in all your patterns, one by one.
Getting the URL of actions with variable path components or GET parameters
When you want to get the URL of a more complex action that has some variable path components like /product/4739, or maybe accepts some GET parameters like /user?userId=381, you pass the parameterValues option key to Request::buildUrl with the values you need.
For example, for this action that has one fixed and one variable path component: