You do it because only WikiTreers who have signed the honour code can edit profiles and, thus, see the hidden text.
You do it because you want the information there (because you, as PM, don't know what tomorrow will bring), but said information might be distressful to living relatives (such as an illegitimacy that nobody knew about, to where the birthdate and place are lied about so the lie is what appears on every internet tree out there except the WT profile).
Having the information there, even if unseen by the majority, means it can be unhidden when enough time has passed that it is no longer as hurtful as it might be today.