Its for the code readability and thereby also the quality:
It was a complex controller method with many lines of code and if/else clauses.
Separating that first if/else into a private method with a name that describes what that construct actually does makes the code much more readable and understandable.

Sandi Metz made some rules, which I find a way too hard and sctrict, but basically that route is good imho.
(http://robots.thoughtbot.com/sandi-metz-rules-for-developers)