Hey there, long time lurker first time poster. I've been playing a lot of Battlefield 1 and I've also watched Saving Private Ryan thus weekend and I noticed the tethered balloons in both settings.
What were their intended purpose? Were they decoys? Did they provide a tactical advantage? What were the logistical requirements for them? Any limitations?
Sorry if this was already covered.
They tended to be used for different reasons.
In World War I (and previous wars), the observation balloon was a common sight. While aircraft could provide irregular reconnaissance, few aircraft carried radios, even in 1918. This meant that aircraft could not be used to radio back corrections to artillery on the ground.
The static battle fronts meant observation balloons were useful. These could be linked via telegraph or telephone cable to the ground and provide near-instant corrections to artillery on the ground.
By World War II, aircraft like the Fiesler Storch (for Germany) and the Piper Cub (for the United States) had supplanted the observation balloon in fast-moving battles.
What World War II did have were tethered, unmanned barrage balloons. These were present during WWI as well, but they became famous during the Second World War as a symbol of air defense. Tethered to the ground with stout steel cables, these balloons were intended to be a deterrent against low-level aircraft. An aircraft hugging the ground, moving quickly, might not be able to dodge a balloon. A collision was usually fatal for the aircraft.
World War II also saw the use of antisubmarine blimps during the Battle of the Atlantic. These blimps could more easily spot periscopes and submarines moving into attack a convoy. These were extremely effective, and I don't know of any successful attack against a convoy escorted by a blimp.