The idea behind having a penciler and an inker working on a page goes back to the most persistent demon in comics: chasing deadlines. If we have an artist who is a really solid storyteller, it makes little sense to have him/her draw each page twice. Better to have someone else handle the inks, so the penciler can do another book in the same time. (This is how Jack Kirby ended up doing so much work at Sixties Marvel.)

A plus, those pencils do not have to be crisp and sharp, since they are not meant for reproduction. It's the inker's job (at least as the job was originally conceived) to sharped up those lines, add distinguishing line weights, shadows, textures, etc.

But by the time I was working my way into The Biz, things had changed. "Soft" pencilers, like Curt Swan, Carmine Infantino or Dave Cockrum were becoming more and more the exception, as young punks like me turned in pencils that were so clean and precise one could cut oneself on the line. (This was a contributing factor in my getting work at Marvel. My pencils were so complete the mantra became "Anybody can ink Byrne.")

Fortunately, my mutant power is that is was sharp and clean, but also FAST. But most pencilers who were turning in work so clean it could be reproduced without inks, if needed, were incredibly SLOW.

This has continued. Artist who can turn out two pages a day (really a necessity with a monthly deadline, at least if the artist wants to have a real life, too) have almost disappeared.