The typical horn speaker is usually made of metal or polymers and can withstand the outside elements better than the cone type speaker, making the horn speaker popular for outdoor PA use. They are far more efficient, you receive more sound volume at a lower wattage, which also reduces both the size and power requirements of your amplifier.
Horn speakers can be best described as "bright" in their sound. The higher frequencies are reproduced very clearly with virtually no bass. The limited frequency response is one reason they are so efficient, they don't waste energy reproducing bass frequencies. The speakers can also project farther without getting "muddy". The end result is better intelligibility at longer distances.