The UK has a lot of 'Indian' restaurants, which tend to be staffed by people from Bangladesh.
The menu will often have 100+ dishes available, which they need to cook using as few staff as possible, so it is my understanding that these will be made from a sauce (which seems to contain lots of onions, then garlic, ginger and spices) usually referred to as a 'base gravy', which the relevant meat and vegetables for the dish ordered will be added to.
Obviously this is an essential process in a restaurant, but there is a large community on social media of British people not of South Asian extraction who want to replicate their favourite restaurant food, and overwhelmingly this involves making a 'base gravy' first.
I live in Indonesia and know how curry is made here, and often the best places to eat might specialise in ONE dish, which is cooked from scratch ahead of time, and indeed any place that's preparing a curry from scratch after you order will tend to have bad flavour.
I'm slightly confused by this 'BIR' system, because most of the gurus (of people who want to make 'Indian restaurant' curry at home) seem not to be of South Asian origin, and nor are the people cooking it. So I'm slightly doubtful about the whole process. I'm assuming that South Asian families would tend to cook curry from scratch in much the same we do in Indonesia.
I can see that it could be advantageous if you want to eat a different curry every day for a week, or if you want to create four different curries and plate (ahem) them into aluminium foil trays to eat at the same time, as if you had just ordered an Indian takeaway. And generally, I suppose, if you create say 20 portions of 'base gravy' and then make 'curry for one', with the other 19 portions in the freezer then it could be useful. But that I think is not specific to curry, it's just meal planning, like people making 20 portions of tomato sauce or whatever else.
So the question is is there really an advantage to this as opposed to simply making a curry from scratch, or is this more a case of slavish mimickry? I presume that this cooking technique is not particularly common among British Bangladeshi people at home, for example.