Apple has recently begun sending out an email encouraging those who opted to stick with an older version of iOS to upgrade to iOS 8. The email details all of the major features of iOS 8 along with instructions on how to install the update both via your iDevice or iTunes.
This email is probably in response to slower overall adoption rates of iOS 8 as compared to iOS 7. Part of the problem was that iOS 8's initial releases were plagued by problematic bugs, so users were more likely to wait until the first dot release to upgrade. Another issue stemmed from the fact that the over-the-air update required a massive amount of free space to install; space that many people weren't willing to free up. To address that second point, this email makes the iTunes upgrade path a clear alternative for those who may not otherwise know about it.