starting with a pencil

[Brad franklin]

When I look at the old master's work on IAMPETH and sometimes new penman they pencil in the work before putting ink to it. My question is when they do pencil it in they always ink it right beside the pencil marks not right on top. I see it all the time. (you can go to IAMPETH and look at artwork and see what I mean) What is the reason for this? The only practical guess would be it would be more erasable because you did not have ink on top of the pencil lead. Can someone explain why?  Many thanks.