Why should design for music be so important for artists? What's the real value in investing in a graphic designer to create an iconic identity for your work. Well the answer lies in that sentence - "Iconic Identity" for your work. Every great album can be associated with an iconic identity, and successful artists have understood and embraced this throughout the history of popular music.
Historically, records were packaged in a plain white paper sleeve, simply to protect the vinyl. These paper sleeves featured little or no printing on them, usually just the music title or producers name. However, In 1938, Columbia Records hired Alex Steinweiss as its first art director. He is credited with inventing the concept of album covers and cover art, replacing the plain covers used before. This new approach to packaging gave columbia's artists a competitive advantage in the marketplace and forced other record companies to follow suite. By the late 40s, all the major labels featured their own colorful paper covers in both 10- and 12-inch sizes using either reproductions of classic art or original designs. Album covers became renowned for being not only a marketing tool but an expression of artistic intent.
Revolver, Darkside of the Moon, Sticky Fingers, Born In The USA, Rumours, Aladdin Sane, Andy Warhol, Led Zepplin, War, Unknown Pleasures, Transformer, X, Nevermind the Bollocks, Is This It, Born This way, Nevermind, Hozier, London Calling . . . all of the above albums (quickly listed off the top of my head) can be instantly associated with an iconic cover design.
Or another way to describe what a designer creates for the musician is a brand for their music. Not a brand in a corporate sense, but in terms of defining a visual identity, the principal is the same. That "brand" defines that album and marks it in time and can be adapted for any related collateral from merchandise to marketing tied directly to that album and its associated tour. Some of the albums I've listed above don't even feature the artist name on the front cover, proving the power of the identity that's created by the album packaging.
In short, the designer creates something that transcends mere packaging. They create a visual representation your music - not all of your music, just a snapshot in time, but a snap shot that last forever.