Surely it matters. When you select your army on one side of the map, to attack the army on the other side of the map, you do not want your army to be stretched out over half of the map, only to be picked off by the enemy one by one.
My impression is that his pathfinding approach basically also features formations.
It’s nice to think it matters now we’ve seen it but how many games to date have not done it? - and were they lesser for it? Just another perspective on making things more clever than they need to be to have fun.
Cas 
And in real life an army advances to a staging point, stops and collects itself, and then proceeds to the assault.
It could potentially matter concerning your own units. It might not particularly matter if the units are skirmishing, but when it comes to navigating groups of units through other groups of units, it might be helpful. Say you have archers in the back and several rows of pikemen and swordsmen in front, and you want to push you archers to the front. Well, without the flow technique, they would have to go all the around the units to reach the front line (or at least you would have to make the units in front create a lane to allow the archers to proceed). Its still pretty interesting.
it is bloody annoying to have your army stretched out to a line in an RTS, and simply is usually not implemented in a better way because it is complex and (therefore) time consuming, both to develop and at runtime. i was aware of this long before i viewed said video, so i do not… concur.
btw: formations in Cossacks were a notable exception, but they cheated somewhat for the complex situations (units would temporarily intersect).
between stops it doesn’t turn into this chaos that most games let these formations get into.
Thanks for the support and suggestions everyone. I applied some the ideas I was given and I’m now able to process hundreds and hundreds of entities at the same time.
Now I can move onto improving the AI, thank you guys again 