Another is to make the image on the front link with the video, if they aren't link people might not make the visual link, for example, if they've seen the music video on youtube, they'll recognise the cover if it looks similar to parts of the video, and therefore possibly draw people in to buy the product.
For example, Ellie Goulding's 'Lights' album cover is linkable to the video because of the use of little lights around her face. And as she is featuring prominently on the cover, we recognise her face too, and it also promotes her and everything she does.
One Direction always feature on their album covers, as it is mainly their apparently 'beautiful' faces people recognise. This 'charm' may help album sales, and promotes the band, as well as selling the album/single. Also their covers are always linked into a video from the album with the theme.