Like @BethBowers I've never found a system enforced limitation or advantage between Raw and FG. However, I direct my clients to use them for logical separation. If an item is purchased for use in the manufacture of a staged or finished good, use the Raw Materials product type. If the item is manufactured and sold to customers, make it a FG item.
The real challenge is when you have staged items that are used as raw material in a bill for a Finished Goods item but is also available for sale. Same for items purchased from a vendor with the primary intent of being used as a raw material to manufacture one or more finished goods but can also be sold directly. In these situations, we either use one or more item categories or UDF's to provide further clarification.