Japan is often considered the animation capital of the world, yet many of the country's most famous works originated in the 2D literary format known as manga. Every year, hundreds of fantastic manga series are optioned as new TV series and movies by Japanese animation studios. Some become popular multimedia franchises, whereas others become dust in the wind. While it's more common to see a famous manga inspire an anime adaptation, the reverse also occurs more than you might expect, and there are plenty of great manga that started as anime.
Manga is more than just a stepping stone for the next big anime hit. Many manga based on anime are used to expand upon the story established by the original series, creating additional content for fans to enjoy. If you only watched the anime, then you're not getting the full picture.