You can find interesting information in "dtv
Atlas Namenkunde" (in
German).
Short summary: It was used in parts of (larger) Germany just to denote the place of origin, no matter of nobility. This use survived until today in the Netherlands and Belgium (using "van" instead of "von") and in Switzerland. In the rest of Germany (and in Austria) von became a nobility marker. The are still leftovers of old non-noble "von" names even in Germany. And some noble families don't use "von", to add to the confusion.