When it comes to URLs, the question mark (?) defines parameters passed to the resource and strictly speaking the result may be another unique resource. Say example.com/search?term=example and example.com/search?term2=example may very well yield different pages (different resources).
However, in the case of static resources, say images, it is possible that example.com/example.jpg?param1=value1 and example.com/example.jpg?param2=value2 and example.com/example.jpg may return the same resource. Still to a browser those resources are different and will be cached on three separate occasions.
Which brings me to why we use this approach: if we change something within the font we want the browser to get the new version. And since we can't really control the caching mechanism efficiently, we simply put another parameter so we can ensure that resource has been downloaded again.
And that's why Alex has suggested removing the parameter along with the ? sign, leaving the bare URL