One theory on these labels is that they were used on letters that were valuable (e.g. included money). They served as a kind of notification to the agents that the letter was noted when it was placed in the system at the originating office. However, I believe that this is an "accepted theory" that lacks firm proof.
While they have a "catalog value," you probably want to collect them on cover. You can usually find them for a reasonable price in bulk lots of Wells Fargo covers at an auction with western postal history, such as Rumsey:
http://www.rumseyauctions.comI have one in my exhibit here:
http://chips.cgpostal.com