First,

.
These were envelopes with an added cachet to serve as the envelope for a FDC. However, they were not used as intended. Many such exist for a lot of stamp issues. While they may have some value and interest to a really, really specialized collector, they are great to use for mailing and certainly make your letter or payment noticeable and eye catching.
Edit:
Belgium was one of the overrun countries for which the USA issued a postage stamp to honor. At a face value of 5 cents, there were intended to be used on mail to a foreign destination. The Philippines was not a country in that set of stamps.

I have attached a photo of a Belgium cachet envelope which was used by the company for mailing third class matter.

Here is the stamp for which the Philippines cachet was intended.

