Administrators have the ability to "protect" pages or images so that they cannot be modified except by other admins (the link "Edit this page" is replaced by a link "View source" when viewed by non-admins).
Pages should be and are only protected when there is vandalism that warrants such protection. In the case of the Main Page that is viewed by many people, I think that's obvious why that's protected. There may also be other pages such as Templates that are used on protected pages, so those templates have to be protected. Also, some project and/or help pages may be protected depending on their importance.
Of course those with the power to do so should be careful that they don't overuse it. What fun would a wiki be if all the pages were locked?