Foundations (whether paper or fabric) are great when .....
1) The piecing is tricky/tiny/delicate/intricate - paper piecing helps with accuracy
2) You are using "uncut" strips like strip piecing. You can use either paper or fabric as a foundation and just add the next strip to the piece and keep going. A lot of strip blocks are made with non-uniform pieces of left over strips and using a foundation helps you manage the size easier.
3) The project will take a looooong time and you want to keep the finished blocks "stable". You can piece them on paper or fabric foundation, baste the final edges and set the blocks aside for another day. The blocks will retain their shapes nicely.