You need to clear it if you want to start with a blank buffer each frame.
Some interesting effects can be achieved by rendering multiple times to the same frame before flipping.
you always need to flip to display the frame.
You could think of it as a double sided chalkboard, the side you're drawing to is not visible to the user but it is to the artist (you) and you need to flip it around to show what you've drawn. then you can wipe the other side clean whilst it's not in view and start drawing again. (a simplistic analogy but I *think* it makes sense)
Some interesting effects can be achieved by rendering multiple times to the same frame before flipping.
you always need to flip to display the frame.
You could think of it as a double sided chalkboard, the side you're drawing to is not visible to the user but it is to the artist (you) and you need to flip it around to show what you've drawn. then you can wipe the other side clean whilst it's not in view and start drawing again. (a simplistic analogy but I *think* it makes sense)