Many CMS, and their editors are a little too complicated for the end-user.
Though many of these are very powerful, and can facilitate many types of changes, for the non-technical user who only wants to make simple content changes, those editors are over-kill.
On-page editing provides a no-nonsense, simple, fool-proof way to make content changes.
Long story short, you just change content as you see it on the screen:
EXMPLE: For the authorized user, the highlighted content of a page are editable. The user can simply type over content.