Among important life skills are developing appropriate filters and maintaining healthy boundaries. Considering the audience and one's relationship with the members of that audience can provide guidance as to how much information one may wish to disclose in any particular situation.
For example, parents on forums have often posted that they encourage their children not to keep a secret from the parents. At the same time, parents may coach their children not to share with others information which they may not like to hear others repeat. Hearing information repeated may feel as though the information is being used as ammunition in a disagreement or in a competitive situation.
If one shares personal medical information with an audience of strangers, that information is no longer private and may be repeated by other students/parents/teachers, etc.
