Why do i need to create a custome profile from a parent profile ?

If I may add, the parent profiles generally have settings in them that have evolved over years of testing and guidance - they are highly optimized. I would highly recommend, that if you have to make ANY change at all to a profile, that you do so in a child profile (to keep the parent profile in its original form).

That said, and more to Josh's point, creating modified profiles is a good way to set standards for different applications (ie. SharePoint compression profiles, etc.).

As for the "mandate", it's really more of a "protect you from yourself" sort of thing, where a child profile will contain the settings inherited from a (hopefully working) parent profile and only require minor adjustments.