It is time we sort this one out once and for all. No, not coffee cups (although we will touch on that), and not bra cups because this is a kitchen thank you very much. Cup measurements for ingredients.
Now it's no surprise I am not a fan, despite my many years in the States, but to leave it at 'Me no like' is not going to cut it, so into the rabbit hole we must go - and it gets complicated (an odd thing to say considering a very simple unit of measure).
And simple unit is pretty much where it starts and immediately ends in so far as listing all the pros for using cups. A cup holds 8 US fl oz (not the same as 8 British fl oz) and differing weights of various dry ingredients, in that different flours can weigh different amounts despite only being '1 cup'. Size and mass are two different things. There is a standard size in the US, but it is feasible to use a coffee cup (told ya we'd mention it) if all your ingredients are in cups, but if we start adding eggs and the like it probably won't be, and it gets further complicated when producing massive amounts of things: a cup may be okay for a cake but isn't when making 1,000.
Didn't stay simple for very long did it? And it's about to get worse. The UK has a cup measurement, as does Australia, and Canada. And the Japanese. And Latin America. And Russia. And ... you get the idea. None of them are the same. So to simply say '½ a cup of XXX' is not enough without knowing the country. And, no, you can't just assume that cup = American, any more than you can assume that Manchester = a town in Tennessee (unless you live there, in which case that's fine, but there's really only a few fast food joints and a giant ass pawn store that sells lots of guitars - I know because I've been in it).
This is where the metric system helps - it's the same everywhere. Doesn't matter if it's 500g of flour, 2km of cable, a 45mm bullet, or a 100m sprint. Of course we're only concerned with the flour here because, again, this is a kitchen, and there will be no running or firing of guns in it.
But how did this all start and why?
As far as American Cooking Royalty is concerned, there isn't anyone bigger than the legendary Fannie Farmer. If you have never heard of her - shame on you - she wrote the now infamous Boston Cooking-School Cook Book in 1896. It didn't just contain recipes but techniques as well and was an instant runaway success - still is to this day - and she is credited with all things cup measurements, she devised whilst at the Boston School of Cooking.
So there we have it Ms Farmer is the mother of the US cup measurement that pervades US culture, and is ingrained into the psyche. Only it isn't because she did no such thing.
What she did was standardise cup measures, because prior to the 1890s a cup was a cup was a cup, so she told everyone to flatten or compact the ingredients meaning it was uniform across the board, and far more accessible to the average woman (because they did all the cooking).
The cup measurement probably was born out of necessity a couple of hundred years prior; if you are out in the wilderness finding a place to call home and needed to whip up some bread, you were unlikely to have scales to hand but a cup of some sort you would.
So there you have it as to why the cup is a thing. Metric has tried to be implemented by the US Govt on a number of occasions but the public just do not want it, and that is despite that guns are metric, the US army uses metric, scientists use metric, NASA uses metric, athletes use it, utility companies use it, and Hollywood uses it to name just a few.
Now where did I put my ¾ cup and my stick of butter ...