Last time I checked, lyrics are part of the music in the same way that different riffs, basslines and drum patterns are part of a band's music. Even if you're listening to something like black or death metal where the vocals are unintelligible, the lyrics are still a part of the music and the way it's presented, and are generally related to the mood or concept of the album.
Couple that with the fact that most bands in the world use vocals that are easily understood, and now you know what lyrics are for.
In closing, even if you personally ignore the lyrics, you'd have to actually be a complete mental retard to seriously wonder why some people find them interesting and appealing.