What is the difference between "class C extends A with B" and "class C extends B" when trait B extends trait A

When two traits are defined like this,

trait A
trait B extends A

what is the difference between these two.

class C extends B
class D extends A with B

I do not think it is necessary for class C or D to extends trait A since trait B already extends trait A.

Why is this often written "class D extends A with B" ?