They actually existed and used by Napoleon.
Wikipedia article on Semaphore
"A semaphore telegraph, optical telegraph, shutter telegraph chain, Chappe telegraph, or Napoleonic semaphore is a system of conveying information by means of visual signals, using towers with pivoting shutters, also known as blades or paddles. Information is encoded by the position of the mechanical elements; it is read when the shutter is in a fixed position.
The system was invented in 1792 in France by Claude Chappe, and was popular in the late eighteenth to early nineteenth century.
They were much faster than post riders for bringing a message over long distances, and also cheaper in their long-term operating costs, once constructed. Semaphore lines were a precursor of the electrical telegraph which would replace them half a century later. The electrical telegraph would in turn be cheaper, faster, and more private. The distance that an optical telegraph can bridge is limited by geography and weather; thus, in practical use, most optical telegraphs used lines of relay stations to bridge longer distances. This also prevented the optical telegraph from crossing long expanses of water, unless a convenient island could be used for a relay station."
The privacy thing is NUTS. People tapped Telegraph wires.
People wanting privacy (on Semaphore or Telegraph) used code systems, cyphers, encryption. Just like some people today. Semaphore in reality wasn't really any less private than email or telegraph.
UK started having copies of all International telegrams (via UK, which was a major hub) before WWI.