According to stats from iTunes, consumers spend most of their money on Apps and Music. "Based on these latest numbers, (Horace) Dediu calculated how much iTunes users spend per year on different types of media. He says it’s 'about $9/yr on Software, $2/yr on books, $16/yr on apps $12/yr on music and $4/yr on video.'” This analysis is culled from his Asymco blog. With videos and books offering the least amount of revenue, it speculates that their is missed opportunity in these buckets.
Clearly iTunes provides a huge revenue and growth opportunity for Apple. Consumer need to upgrade and add more content to their devices drives purchases and the more devices connected to iTunes the better. According to the site, iTunes currently has "575 million active iTunes accounts". But if each account is like mine, a family with multiple users on the same account, that means that there is possibly 4x the number of users per account, each actively purchasing and downloading. Add to that the number of devices and the need for content continues to increase.