Why are there 3 modes of editing a page?

For convenience. It works fine when your pages are very long and editing the whole content is not always the best solution.

So the modes are:

• whole page edit
• section edit
• append

Each of these modes introduces a page edit lock such that no Users can both edit a page at the same time of their locks conflict. But as you might expect different users can edit non-overlapping sections of the same page at the same time. Or edit a section and append. These locks do not conflict.

Anyway - with long pages it is much easier to use the section mode or append mode than editing the whole long page.