Typically brown field analysis is a result of a desire to make modifications of existing piping or analyzing the effects of "mishaps."
As a rule of B31.3, you need not pick a pipe with successful service life and analyze it for no reason, but there may situations where you might, anyways.
An example concern with brownfield design you might consider is the tendency of (some) owners to maximize service life by increasing the corrosion allowance based on pressure, and subsequently derate the pressure. However, this means that your (particularly) small diameter piping approach the thickness of soda cans, while still great at holding a bit of pressure, are not structurally sound enough to hold up those valves and piping spans.
If they are inspecting regularly, and they note pipe thinning, sagging, leaking, etc, then they stand to benefit from analysis to stop those, and prevent future such problems.