Placards can be there to give an operating restriction before an AD is complied with (Citations have a placard stating "500lb fuel minima per side" which will be removed when a mod to the fuel pumps is carried out).
They are mandatory in this sort of case.
Another one which most will have seen is "GPS eqpt VFR use only" or "Radio eqpt Class 3 iaw AWN84". The same goes there.
No addition to the POH is required in either of these cases, yet they are there in accordance with legislation. The POH only details the basic level of placarding required when the aircraft is delivered.