Most modern day backpacks are water resistant but not waterproof. They might keep their contents perfectly dry in a light shower but prolonged or heavy rain will eventually soak in (either through the material or through the zips) and your clothes, maps, and food will get damp. Nobody wants that- a wet jacket is no use if it gets cold and a wet sleeping bag is one of the worst things a hiker could hope to avoid. Happily, rain covers for backpacks are lightweight, adaptable to a wide range of different bag styles, and simple to deploy even in the worst weather.
Equinox Gila Ul Horizontal Pack Pocket LIBER145692