Iwa ni Hana asks:

Why would fans want to experience / creators want to tell more or less the same story with more or less the same characters in different formats, be it manga, TVA, OVA, feature film, CD drama, novel, live action movie or live-action TV series?

Two answers, I think. Firstly, we're talking about different sets of people which intersect but are not equal or belong. For example, I essentially read no manga, play no games. So, every adaptation (such as ef) is a new experience. In addition, formats may add something of value, for example motion and music are absent from manga. So, I saw the Peter Jackson's Lord of the Rings movie after reading the book (several times), and it was money well spent. This way, formats form a vague hierarchy of expense, with cheaper works (such as manga) forming the base and being adopted into more expensive arts. So, when adaptation happens against the hierarchy, generally only new fans would want to experience it (e.g. anime to live action). I saw "generally" to forestall the example of a die-hard fan who buys manga to complement his favourite anime.

UPDATE: Kaedrin comments.