You got excellent responses. There is a much simpler reason to add to the list of why you should avoid using these "compound" formulas.
You might want to reference the inner formula (in this case Item () ) from another line item, so if you separate it you will easily have access to it. Also, it's a matter of getting used to writing a simpler formula for Auditability PLAN. Finally, Some functions won't allow you to use a compound formula as a parameter. For example, you can say Source.line item[lookup: parent line item] where parent line item = PARENT(ITEM(L3)