From my experience of the MM's three battery charging hub,
Qu) "Why? It is very big and not compact at all. "
Ans) But it acts as a holder/storage-vessel/protector for the three batteries it can charge, I like that idea very much. In so doing it is not much bigger than the volume of the batteries themselves.
In your other thread you imply buying 3 of these "usb battery charger"s, so, will the volume of 3 loose batteries and three of these "usb battery charger"s be significantly less than that of the DJI triple charger containing 3 batteries?
Point) "It can charge only 1 battery at time and it is cost more then 3 times then this usb battery charger."
Ans) True but one of your "usb battery charger" things can only charge one battery at a time and, since you imply buying three, that would be £38.04 going by UK prices (£12.68 each according to the page your link takes me to). The DJI hub is £39 + carriage, not a vast difference and if the DJI charger could be found on Amazon I suspect it would, at £39, be carriage free.
Reading the specs of your "usb battery charger"
I see
"Charging Time: QC3.0/5V3A-about 80 mins, 5V2A-about 110 mins. "
which makes me wonder what the charge time will be when powered by your "anker powerbank with 3 output usb slots"?
It may be an even more relevant question if you are attempting to simultaneously charge 3 batteries from your anker powerbank.
The DJI hub seems able to charge 3 batteries in 4 hours (80 min per battery) using an 18W charger but can accept a 29W which I guess means a faster charge. see
Buy DJI Mini 2 Two-Way Charging Hub - DJI Store
I don't know the output of your anker powerbank.
Make no mistake, I think the "usb battery charger" have their uses but maybe the advantages are not as clear cut as you imagine.