Music videos and album covers can define a band, but neither is as important as the music underneath. That said, album covers and videos are meant to define the tone or theme of a song or collection of songs. Because covers became important, The Beatles started to really think about the look of their artistic labels.
Once brown paper and cardboard sleeves were no longer the norm, bands started to think of the packaging as representative of the band, versus just a protective cover. Cover art started in the late 1930s and record stores had to be redesigned, in terms of layout, to show off the new covers.
Music, while being representative of the soul, is also a product. Basically, a record cover represented the special dimension of a band or artist. The Beatles were more than aware of this as they changed up their covers. They used the art as expression and to showcase deeper meanings in music.