When a piping line specification requires 6mm of corrosion allowance, do you consider necessary to indicate this requirement also for valve description?
Considering that valve's body thickness (according to main standard: API, BS, ASME) is greater than the pipe gate wall thickness (including 6mm of c.a.)it would seem unuseful.
The standards that govern valve shell thickness generally have corrosion allowance built in.  Some parts of a valve have essentially NO corrosion allowance: as soon as the seating surfaces corrode ANY AMOUNT, the valve will leak.  It can be dangerous to throw in a number for corrosion allowance that conflicts with accepted standards.  If you ask for an additional 6mm corrosion allowance on a DN16 valve, vendors might decline to bid, or they might charge you for all new patterns/engineering/ special castings which all add up to mucho dinero. 
"Anyway sometimes this is required by Client/Licensors in Oil and Gas Downstream Business.
Does anybody know why?"
If I were cynical I would point out that Architect/Engineer firms get paid by the page in their specification. However, there is the thought that they might want to ensure a certain amount of maintenance-free service life.  

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