When I saw many sites' source code, parameters were passed to the linking file (CSS/JavaScript).

In the Stack Overflow source, I got 

<script type="text/javascript" src="http://sstatic.net/js/master.js?v=55c7eccb8e19"></script>

Why is master.js?v=55c7eccb8e19 used?

I am sure that JavaScript/CSS files can't get the parameters.

What is the reason?

javascript css parameters hyperlink
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