It is meaningless to you because you have no use for that information which is probably why she shared the "super super secret" diary with you. In terms of your relationship with your friend, you present a harmless consumer of that information.
As a though process, lets replace "you" with:
a) her employers, colleagues, etc
b) any of her family members that she talked about
c) her significant other, partner, etc.
Now, do you think she will let these people read her diary? Do you see how dangerous these "secrets" are if these parties get their hands on it?
IMO there are always valid or invalid reasons for keeping secrets (privacy is actually the real topic here) but the real question is whom are you keeping this secret from.
As a though process, lets replace "you" with:
a) her employers, colleagues, etc
b) any of her family members that she talked about
c) her significant other, partner, etc.
Now, do you think she will let these people read her diary? Do you see how dangerous these "secrets" are if these parties get their hands on it?
IMO there are always valid or invalid reasons for keeping secrets (privacy is actually the real topic here) but the real question is whom are you keeping this secret from.