It was theorized on Hobby Killer's blog back in the early days of
AoS but I don't think it's something anyone really uses or provides much benefit.
The idea is as Solosam says. It kind of gimps your own ability to charge with as many guys as possible, it will affect your enemy only if he rolls just low enough to make it into contact with the very closest model, if you're forced to go first in that combat and pile in then the effect no longer happens, and if your T is that spread out, you're not going to get that close for the pile in either.
There's certainly a lot of importance in setting up your formations and charges for maximum effectiveness, but I don't think this particular idea caught on anywhere after being mentioned early on. Unless I'm mistaken.
https://hobbykiller.wordpress.com/2015/09/08/form-up-for-success/