Hrolf is right, session musicians often fill in and take lead in recording sessions, for any number of reasons and the process of recording can be very different than playing live.

Also there is a financial element - The art of a good session musician is the ability to get it right first time or second time. Where as people I know who are in bands are terrible 'fiddlers' of music when they're recording their own stuff. From a record companies perspective you don't want a band to be 'fiddling around' with tracks when you're paying for studio time - and a good session musician will hear a song once, be able to play it back perfectly, whilst the writer is still mastering the song.

Of course by the time they go on tour, you've probably mastered the song entirely anyhow.

Hrolf is probably right, that session musicians often rank among the unknown heroes of music, along with producers, and their talent goes unnoticed - if not unrewarded (hell some professional musicians). Sometimes, they even form bands themselves (If I remember rightly Garbage were formed from session musicians and production staff members, and I think Led Zeppelin might have met that way - Page was a session musician firstly).