It's totally reasonable to assume that each member of a band writes his or her own parts: the guitarist writes the guitar parts, the bassist writes bass lines, and the vocalist writes the lyrics. However, there are some rare cases where someone in the band other than the vocalist writes the lyrics. Here are six examples of bands whose vocalists don't write their own lyrics.
1. Liam Gallagher - Oasis
Liam Gallagher originally joined Oasis without his brother Noel, but when Noel saw the fledgling group perform live, he felt they could be world famous if he was put in charge. Noel joined Oasis as its lead guitarist and primary songwriter, leaving Liam to front the band and sing his brother's lyrics.
2. Roger Daltrey - The Who
Back when The Who was just another R&B cover band from London, lead vocalist Roger Daltrey was considered the group's leader. However, when guitarist Pete Townshend began composing original music and lyrics to attract label attention, he soon became the group's primary creative force. This became most apparent to Daltrey in 1965, when he was kicked out of the band and only allowed back in when he agreed that The Who was no longer his to lead.
3. Geddy Lee - Rush
Neither bassist/vocalist Geddy Lee nor guitarist Alex Lifeson of Rush were ever interested in writing lyrics for its songs. Though the band's original drummer John Rutsey had intended on being chief lyricist, he never submitted any, forcing Lee to quickly write some for the band's eponymous debut album. When Neil Peart eventually replaced Rutsey in 1974, he actually delivered lyrics for Lee to sing.
4. Ozzy Obourne - Black Sabbath
Ozzy Osbourne is by far the most famous member of Black Sabbath, but he's not as integrated into the band's songwriting process as you may think. Though Ozzy writes the vocal melodies for the songs, guitarist Tony Iommi is the band's primary music composer, and bassist Geezer Butler writes most of the lyrics. Osbourne would go on to write his own lyrics for his solo career (though with occasional outside help from people like Lemmy Kilmister).
5. Dave Gahan - Depeche Mode
Depeche Mode has gone through two different primary songwriters during its career, neither of which was lead vocalist Dave Gahan. Its original songwriter was Vince Clarke, but after Clarke left the group in 1981 and went off to form Yaz, keyboardist Martin Gore took over his role of writing music and lyrics. Most of band's most famous songs were written solely by Gore, though Gahan would contribute to songwriting beginning in 2005.
6. Any of King Crimson's '60s/'70s Vocalists
Guitarist Robert Fripp may have been the face and driving force of progressive rock band King Crimson, but he never sang any of the band's songs. A frequently rotating cast of bassist/vocalists, including Greg Lake, Gordon Haskell, Boz Burrell, and John Wetton, served that role. However, neither the vocalists nor Fripp wrote the band's lyrics. That job would go to Peter Sinfield and later Richard Palmer-James, neither of whom actually performed in the band. It wouldn't be until 1981 when vocalist/guitarist Adrian Belew joined the line-up that a King Crimson vocalist would write lyrics.