As I recall, this option has existed for tripleA mods since at least 2011. It was a phase order idea implemented in the tripleA game NWO, and discussed as a general idea for larger maps in TripleA even before G40 came out. I can remember reading a conversation about this stuff with Comrade Kev and Veq a few years ago. I imagine there is probably a tripleA mod of this by now for G40.2 floating around somewhere.
I think the general principle is sound though, since many of the delays you encounter in game, have to do with purchasing decisions. Or, as is sometimes the case in large maps with very involved turns, that players buy something, then forget why they bought it, and make a mistake somewhere. This leads to yet more delays, as they then have to replan everything during Non Com to try and salvage their botched purchase haha.
In FtF games what this sort of playstyle prevents, is the situation where players conflate the Combat Move and Combat Roll phase. So for example, a player says, “OK well I know I want to do this, so lets run this battle real quick” and then after seeing the results they decide “Oh wait, I forgot to run this other battle, so lets do that now.” This happens a lot in FtF games, where you don’t have a computer to enforce the strict separation of phases. Here what you do is insert the Purchase phase between them, so there is this natural pause in the action, and then the rolls. It gives a really nice pacing to the flow of the game, a more accelerated pacing.
Of course in tripleA, there is also a problem of conflating combat movement and non combat movement (since the engine doesn’t enforce this). If you play tripleA you’ve probably seen this happen quite a lot, or been guilty of it, or made a mistake because of it yourself. By sneaking the purchase phase between combat move and combat roll, you also help to delineate Combat movement from Non-Combat movement, and encourage players to wait until non combat to make non combat movements, which can also help.
Basically what it does is help to prevent a lot of common mistakes that players make as a result of the purchase position in the phase order, and shifts that purchase phase to a position where you’re less likely to make mistakes. Less time consuming mistakes = more time spent on the action, or at least I believe that’s the idea behind it.