Postal history collectors tend to value commercial covers over philatelic covers.
For example, take the 1938 Presidential stamps, the Prexies as they are called. Some of those denominations did not meet a specific postal rate. Thus, solo usages on a cover are rare and highly sought after. A philatelic use though is usually worth much less.
Here is the rub though. How do you know if a usage is philatelic or not? It is harder to determine than you think.
Some covers are very obvious. They have a return address of some long ago stamp dealer or the name of some well-known collector.
Some covers are obvious from the stamps used. A cover with a single stamp from each of the 1901 Pan Am issues that clearly overpays the postage from post office A to post office B is almost certainly a philatelic usage.
I contend that some covers are not so obvious.
Take Scott #2481, the 45˘ Pumpkinseed Sunfish released in 1992. Good luck trying to find a solo usage of that stamp properly used on cover in the 1992-1993 time range.
Suppose the year is 2067, fifty years from now. Going through a dealer’s $3 box of ordinary covers (inflation, you know!), you find such a cover. It is addressed from Bob Smith in Albuquerque, NM to Jim Jones in Charlotte, NC. It has a normal postmark and nothing else. A rare cover like this is worth $250. But is it a philatelic usage?
In 2067, Bob Smith and Jim Jones are either dead, or not living at those addresses any longer. Contacting them to see if either one was a stamp collector is not possible.
You go back into the records of the APS or other stamp societies around in 1992. Was Bob Smith or Jim Jones a member? Nope!
What if Bob Smith and Jim Jones were just two solitary stamp collectors? Neither one had a million dollar collection that ended up in some named auction sale. Neither one was interested in joining any stamp clubs or reading any philatelic publications. They were happy collecting the stamps on their mail and that is it. Bob just decided to use the Pumpkinseed Sunfish stamp to his buddy Jim because he thought Jim may need one for his collection.
How are you going to know if that is the case? Were Bob and Jim possibly related through family and this was just some arbitrary letter between two otherwise ordinary people?
What if the return address was for a business instead? Today’s business mail is almost always a postage meter or a computer generated stamp. Back in the 1950s and earlier, postage meter stamps were not as common. It is possible that a stamp collector owned “Joe’s Gas Station” in Harrisburg, PA and put a solo use Prexy stamp on his outgoing mail. The corner card is for some long forgotten business. Is this a philatelic use? Is it a commercial use? How do you know?
Some covers are dead giveaways as philatelic in origin. There is just no way to be sure though on some other covers that appear to be ordinary. Some collectors pay big premiums for seemingly commercial covers. It is quite possible that they are paying for just another philatelic use too.