Digipaks advertise the artist by showing what the artist looks like or what the style and music of the artist is like. First albums always show the artist clearly to introduce him to the public, next albums can be more creative and free. Jack Johnsons albums are examples of that:
His first album (I) shows a mid shot of the artist and his name is written in a big clear letters with no album name. It clearly introduces the artist and his name. The second(II) and third(III) album still shows the artist but he's less important in the design and the font is much smaller compared to the first album, drawing the attention to the design and not the artists identity.
