There is quite a significant difference between being able to see someone's posts (or an individual's postings) - which, obviously, are public, once you have pressed "post reply", for that is the whole point of posting something - and whom you permit to be able to see, or view, your profile, which you can choose to limit access to, as per the settings of the site.
The question asked in the OP is about members who choose to limit who can view their profile.
And to choose to limit who can view your profle, (for people get to set their own boundaries) does not, in any way, contradict the fact that whatever you choose to post can be viewed publicly.
Almost all of us here have chosen to post under a name, a nom de plume, which is not the name by which we are known in the real world.
That is a form of privacy.
Limiting who can view your profile allows you to set boundaries, not just in terms of what you have written about yourself for public consumption, (and remember, any platform where access is free means that you - the member of the forum - are the product, whose data may be of interest to advertisers), but, also, to set limits, in terms, of who can "follow" you (if such a thing is of interest to you), or, even more pertinently, it allows you to set boundaries on who can contact you privately, by DM, (other than the staff, who - again, for obvious reasons - must be always able to contact you).