Their hit "We Are Young" has become an anthem for millennials and took home the Grammy Award for song of the year at the 55th Grammy Awards. It was covered by the show "Glee" (whose version of it hit number one in the iTunes store before the band's) which made it hugely popular and then used in a 2012 Chevy Sonic commercial which made it hugely lucrative.
And then they dominated the Billboard charts at number 1 for six weeks straight. So they had a big, excited crowd to see them and openers Tegan and Sara when they played at Pier 26.
The show was a spectacle with lights, costume changes, and wild fans.