Quote Originally Posted by Acorn View Post
Why use a Layer when you can Lock the Shape and trace over it?
Also if tracing ensure the final trace shape is partially transparent so you can see and correct errors.

Acorn
if you want to be able to manipulate all your tracings without grouping anything - eg hide/show all in one go - you really need tham all on their own layer - eg line art from pencil scans

I can think of a few other [non-web] reasons too relating to colouration techniques