It's been culturally significant since 1939 and still is. There are still packed shows there almost every night. Music is one of the few places Seattle really has culture.
>Founded in 1939,[4] the Showbox has hosted a diverse offering of music over the decades. From the Jazz Age to the Grunge Era, the ballroom has featured shows by Duke Ellington, Muddy Waters and the Ramones — as well as local artists such as burlesque performer Gypsy Rose Lee, and grunge bands Soundgarden, Pearl Jam, Mudhoney, TAD and Screaming Trees.[5]
>Other acts to perform at The Showbox have included Al Jolson, Mae West, Nat King Cole, Jimmy Durante and Dizzy Gillespie, The Police, XTC, X, Blondie, Gang of Four, Iggy Pop, Devo, Dead Kennedys and Jerry Cantrell.[7][8] More recent performers include The Weeknd, Snoop Dogg, Dave Matthews, Kanye West, Lorde, Robbie Williams, Modest Mouse, Death Cab For Cutie, Public Enemy, PJ Harvey, Wilco, The Flaming Lips, Daft Punk, Kasabian, Old 97's, Elliott Smith, Peter Murphy, Guided By Voices, Built To Spill, Billy Idol, David Bazan, Bebel Gilberto, Cat Power, Spiritualized, Sleater-Kinney, Minus the Bear, Coldplay, Bright Eyes, The Roots, Sir Mix-A-Lot, Dita Von Teese, Maroon 5, The Shins, The Melvins, My Morning Jacket, LL Cool J, DJ Shadow, Scissor Sisters, TV On The Radio, Ke$ha, Kimbra, Marina and the Diamonds, B-52's, Lady Gaga, Ice Cube, Paul Simon, Macklemore, JoJo, (Kpop group) A.C.E and many more.[9][5]
https://en.wikipedia.org/wiki/The_Showbox