1)Davies will respond for you : "Immediately asking White's bishop this question make a lot of senses, altougth it has been frowned upon by successive generations of writers for encouraging White to play a move he'd like to make anyway." So we can say that it is a sort of testing move of the pair bishop against doubled pawn I think..

2)I suppose that elspringer ask to players who have read the book, if they did they will know what he is speaking about.

To elspringer: Maybe the "slow buildup" is ok because the game is positional now. But I still like 5.e4
But more practice is needed...

I agree with Davies that 5. e4  doesn't give much and that the whole plan of 5. e3 & Bd3/Qf3/Nge2 with pawnstorm can easily be spoiled by ... f5. (One of those instances where doubled f-pawns can be put to good use...).
On the other hand, his proposed slow buildup with the double fianchetto is... well,  slow.
Does anybody have an alternative ?