Conversion is the tireless errand of software development. Most programming tasks boil down to some variation of transforming data into something more useful.
In the case of user-facing software, converting data into human-readable form is an essential task, and a complex one at that. A user's preferred language, locale, calendar, or currency can all factor into how information should be displayed, as can other constraints, such as a label's dimensions.
All of this is to say that sending -description to an object just isn't going to cut it in most circumstances. Even +stringWithFormat: is going to ultimately disappoint. No, the real tool for this job is NSFormatter.