I meant to polite, Oomis.
There is one purpose. To symbolize a woman’s married/has been married status.
However, even if it’s purpose is not tzinius, it still must be tzinius.
A nurse’s white uniform is not for tzinius. It symbolizes her status.
It still must be tzinius. Not more or less then had the nurse been wearing a regular item of clothing. Or jewelry. Or anything.
A woman’s shaitel must be at least as tzinius as an unmarried girls hair. Simple.
That is an absolute fact of Halacha. Just like a nurse’s uniform.
A single girl of 45 – may there be not such thing in Khal Yisrael! – lacks a knowledge and dignity that a young married woman has. Unfortunately, there are aspects of life , relationships, family – that she will never understand.
For the record, all Poskim agree that there is an obligation to wear a shaitel after a normal marriage. The dispensation you heard of is from R’ Moshe, who permitted dating women to present themselves as not having been married previously (obviously revealthing their real status before engagement etc.) because of the tremendous hefsed (loss) it would have caused them to cover there hair. The obligation still remains. Of that, no one disagrees