I've often wondered if there is some business principle of publishing that makes it advantagous to continue an existing publication(volume and issue #) under new title, rather than cease the existing title and create a new publication at #1. This has been going on as far back as the 1950's, maybe even the 40s, so I don't think it's strictly a marketing ploy to get out issues 100, 500, etc. sooner and make a big deal about them.
I'm curious if there was a "publishing industry" reason for DC to continue My Greatest Adventure as Doom Patrol with current issue number rather than discontinue the former and put out Doom Patrol #1. Doesn't silver age Flash start with #104? If so, why wouldn't Marvel have gone with Sub-Mariner #102 and Hulk #7 in 1968 expansion?
Just something I've always been curious about. Isn't there some type of publishing number that stays with a magazine regardless of title as long as it remains the same volume and continues existing numbering.

rw2516, if I'm remembering right the rest to change the name but keep the numbering going was more based on the stores that would carry the books. Comics started off like magazines, disposable. The stores didn't want to take a chance on a first issue since it wasn't a tested issue, so to be able to keep selling issues they would keep the number. In some cases that would start with a random high number (don't remember were I read that). By about the late 60's - early 70's comics became more collectibles.

Nah, stores didn't care. A newsstand would get a stack of comics from their distributor and put 'em out. I doubt the typical store that sold comics prior to the direct market could tell you the difference between a Fantastic Four and a Superman, let alone cared about a #1.

There were two things going on with the numbering.

First, there was a perception -- right or wrong, I don't know -- that kids wouldn't want to take a chance on a low-numbered title. Flash #1? Who the hell knows what that is and whether it's any good. But Flash #105? Well, that's been around for a long time, it's obviously a good enough book that people have been buying it for more than a hundred issues.

Second, comics needed to have third-class mailing permits for subscription copies. So when it was time to bring out a new book, rather than go through the paperwork (and fees, I think) of getting a new permit, you just transfer over the permit from the old book by pretending that the new book is a continuation of the old book. And you do that by continuing the numbering of the old book with the new book.