Some times dim's on prints are easier to machine or manufacture, than it is to Inspect., so for manufacturing a ref dimension can be used to verify that the part is being fabricated correctly. It is depended on the type of product & the method of manufacture.
It helps the Inspector or the fabricator as a second verification.
It is not given a tolerance. however ref dimension can also be dangerous if it is the incorrect dimension.
so use them sparingly as not to cause issues.
I use them often for certain circumstance in manufacturing,to aid manufacturing & Inspection.
