Answer by taking a quote from the following article:

The Wallflowers are an American rock band formed in Los Angeles in 1989 by singer-songwriter Jakob Dylan and guitarist Tobi Miller. The band has gone through a number of personnel changes but has remained centered on Dylan. After releasing their eponymous debut album in 1992, the Wallflowers released what would become their best-known and highest-selling album, Bringing Down the Horse in 1996, which featured songs such as "One Headlight" and "6th Avenue Heartache". They went on to release an additional three albums before going on a seven-year hiatus, beginning in 2006.

In July 2004, the Wallflowers returned to the studio to record their fifth album, Rebel, Sweetheart. This time the band decided to record in Atlanta, Georgia, which is where their producer for this album, Brendan O'Brien, is based. O'Brien also contributed on guitar. Fred Eltringham joined the Wallflowers as their new drummer. Jakob Dylan wrote the songs, of which keyboardist Rami Jaffee has said: "What I did notice is that kind of upbeat song with some pretty scary lyrics." Dylan painted the album's cover art himself.  On October 14, 2004, the Warren Zevon tribute album Enjoy Every Sandwich: The Songs of Warren Zevon was released, on which the Wallflowers covered Zevon's 1978 song "Lawyers, Guns and Money." In promotion of the album, the Wallflowers performed "Lawyers, Guns and Money" on the Late Show with David Letterman with Zevon's son, Jordan, on October 12, 2004.  On October 31, 2004, the Wallflowers were flown via military transport plane to the USS John C. Stennis aircraft carrier in the middle of the Pacific Ocean to perform for the returning troops.  Rebel, Sweetheart was released on May 24, 2005, and was met with positive reviews. Despite widespread critical acclaim, Rebel, Sweetheart performed relatively poorly commercially, peaking at No. 40 on the Billboard 200. However, the first single from the album, "The Beautiful Side of Somewhere", hit No. 5 on AAA radio. The second single was "God Says Nothing Back". This was the first Wallflowers album to be released on DualDisc. On one side was the album, and on the other was a DVD that included exclusive performances and arrangements of some of the band's songs, as well as an interview with comedian Jon Lovitz. In promotion of the album, the Wallflowers did concerts for the Oxygen Custom Concert Series and PBS Soundstage. Around the time of the album's release, the band set out on what would be their last tour for two years. They were joined by Stuart Mathis on lead guitar. After 2005, the Wallflowers ended their relationship with Interscope Records.

Why did Stuart join?