Creating content for the web requires webmasters to be jacks of all trades. We must be able to work with server configurations and software, install and write web applications, structure a site, design a visual layout, and write content. With your focus split in so many different directions, it can be hard to zero in on any one area when you really need it.
Simple allows you to separate content management and authorship from file maintenance and visual design concerns. Want to make a new page? Just go to where you want to place it, click "create child page," and start typing. Want to see what was on your homepage last June? Just go to your homepage, click "revisions," and view the page from the desired date. Want to move your "contact information" into your "about us" section? Just go to the "contact information" page, click "relocate," and select the "about us" section.
There's no need to move files, create folders, fix links, or dig through backups.
After all, why complicate things ... when it could all be so simple?