Prior to the issuance of Germany's first official airmail stamps in 1919, airmail stamps were privately issued for mail which was carried on flights by German airships ( or "Zeppelins"). Before 1912, the Germans had developed sophisticated luxury airships, which transported passengers and mail. The stamps issued by the airship companies paid only the carriage fee for the airship company. Letters carried by, or transferred to, the German postal service had to have regular postage stamps affixed to them, as well.
In addition, mail was sometimes carried on pioneer airplane flights. Airplanes of this period were still very experimental and prone to problems. The stamps used on the mail carried by the pioneer airplane flights were issued solely to finance these events and had no postal value. They were issued solely for sale to collectors, and were sometimes affixed to postcards carried on the flights.
The Scott Catalog does not list these stamps because of their unofficial status, though Michel does. Despite the fact that they probably should be considered labels or Cinderellas, they are very popular in Germany and among Aerophilatelists worldwide, and may be seen as interesting relics of the early days of airmail delivery. It is likely that the worldwide interest in the Aerophilately topic will grow more rapidly than the demand for German stamps, and based upon those trends, as well as the low printings of some of these stamps (as noted in the StampSelector Scarce Stamp Quantities Issued List under Germany), I consider them attractive investments.
Those interested in learning more about investing in stamps are encouraged to read the Philatelic Investment Guide ($5), available on Kindle, and accessible from any computer.