Content models sometimes get large and unwieldy over time – lots of types, with lots of attributes. Remember, content models need to be manageable. I’ve seen models with multiple dozen content types, each with dozens of attributes many of which are repeated on multiple other types, and the whole thing is so complex that editors have little idea of what content goes where and types are poorly differentiated from each other.
Two features can help you manage this by centralizing attributes into manageable units which can be combined to form types.