The + is a reserved character for the query string's replacement for spaces.
%20 in a URI is how a space is represented there.
In the case where I have a client (Welcome - Wilderness Wally's Americana!) who wants to use the title of his article as the URI, I convert those spaces to _'s BECAUSE the more common -'s can also be found in the text of the title. Since there are no natural _'s in text, they can be inserted and restored with confidence. An additional benefit of _'s is that, visually, they let the words stand out just that little bit more. For domain names, however, _'s are frowned upon (reason unknown - unless it's typing).
Sorry folks, I've had this explanation in my tutorial article for years (although your search list didn't include the Article).
Mitt - SPOT ON with Sir Tim Bernews-Lee's treatise on the URI Generic Syntax!