Not every person who writes songs is an expert in all phases of songwriting. Sometimes a songwriter might be a better lyricist, and other folks might be more talented at the music writing part. Or, it may just be that a beginning songwriter can be paired with a more experienced songwriter to craft a stronger song (and a better songwriters!)
In Nashville, co-writing is not only a time-honored tradition, but a publishing mandated necessity. Pairing strong writers together creates amazing songs with strong verses, choruses, ideas, and structure. And it makes each writer better for the experience.