The blimps were developed to serve two purposes. They were used to observe movements in the enemy trenches. The second purpose was to keep the enemy airplanes on a safe distance so they wouldn't be able to strafe your own trenches. The blimps in Fields of Battle are used only to help the enemy airplanes from strafing.
<l:h1>Technical information:<l:/h1>
Effect: One blimp will take a succesfull hit from one enemy airplane. This means that the blimp is destructed and the enemy airplane failes to hit your other units. If an area containing blimp units is conquered the blimps will also be conquered.