I am [almost] a professional Scratch animator. I used to animate using backdrops which were my 'frames', but since I made a 800 frame animation and Scratch crashed, I've been using sprites and their costumes. This is because it is more efficient.
Credit to: Alan Becker [Two years from now on July 24th, it's Alan Becker's 20th anniversary on YouTube! Comment, 'Happy Anniversary' below!]