The barometer is one of the greatest fishing tools in existence, and also the most overlooked. Fish may not know how to read a barometer, but they certainly know what the atmospheric pressure is doing.
When the pressure swings wildly (example- just before a storm) most fish get very active. When the pressure begins to drop (example- as a front approaches), fish go on the feed. When the pressure is high and steady (example- high bluebird sky days after a front has fully passed through), fish slow down and are not overly active.
Pro Tip: With so many good weather apps available for smart phones, it is no longer necessary to keep an old school barometer on the wall (Although it is still cool to have one!) Keep a good weather app on your phone, and keep an eye on what the pressure is doing!