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The Webway is a transport system of the Old ones. The Eldar use the remains of that sytem of gates to travel.
Recently a Map has been shown, along the Harlequin release, of the Galaxy and the gates.

GW blog:
http://www.games-workshop.com/resources//blogs/2015/02/18/7MNP4L5XPBCDBYMX6OQKP4RTSWUUUTBN/2xl.jpg
Dakka News&rumors post by Warhams-77:
http://www.dakkadakka.com/dakkaforum/posts/list/1800/628189.page#7603586

Its just a small glimpse of the Map. Can't make out a lot of the names
.....but it seems the Human worlds with webway gates are all Former Home to Space Marine Legions.

I'd like to know if there is/was some fluff explaining the method and reasoning behind the placement of the Webway "access points" ?

Should be the other way around, since those would have been Eldar worlds long before Humanity crawled out of the primordial ooze. Why would Humans build worlds near Eldar Web-gates?

The answer probably being "they didn't know they were there".