Notes databases were easily replicated for offline use and encrpyted for data security (when I was using it was back in the mid 2000's before Full disk encryption became common).
In the days of poor Internet connectivity this made it immensely valuable for travelling consultants who could easily take large quantities of information on the move with them and synch it when in the office.
These days for end users I guess OneNote comes closest to that, but even there I don't think it has all the features that Notes did...