I read somewhere that Infrared film was used in WW2, and later, for aerial and reconnaissance photography. A recent experiment I did, shows clearly why:
These two shots are taken at the same time from the same spot using the same lens. One is with a normal DSLR camers, the other with an IR converted one (780nm filter).
As you can see the difference in detail is quite dramatic. Infrared light penetrates much better though fog and mist and allows a lot more contour detail to be captured.