If Bar inherited from Foo, Bar would have Foo's methods. So if you implemented Bar for Gum, Gum would get Bar and Foo. But Bar requiring Foo means that if you

If Bar inherited from Foo, Bar would have Foo's methods. So if you implemented Bar for Gum, Gum would get Bar and Foo. But Bar requiring Foo means that if you want to use Gum in a place that expects Bar, Gum must have both Bar's methods and Foo's methods. In some cases, you might be able to derive some of those.