a 1:1 ratio of minerals to industry seems a bit boring to me...might as well just combine them into one resource if that's the case.
So you could if that was the entire story.
But the whole idea of separating the layers of the production chain is that you can give the _player_ more freedom in the form of being able to change the ratios from the standard "1 mineral equals 1 industry equals 1 production".
(This example only illustrates what you _could_ do in the game by having separate mineral, industry, and production points. The hard figures and functions are just used to illustrate the process.)
Your mineral output is 100 points. You have built industry that's currently worth 60 industry points. If you let it at default and you have enough people to work your industry your factories would work at 100% efficiency (i.e. fully staffed). The industry would consume 60 mineral points and produce 60 industry points (think of it as "capacity" or production potential that can be used to build something useful). The remaining 40 mineral points are your surplus (stockpiled, sold, wasted, whatever). If you had been short of people you wouldn't have been able to use all of your industrial capacity, e.g. with 50% of the required labor you would have processed only 30 mineral points to 30 industry points but still paid the maintenance cost for your full 60 point industry.
The minerals to industry part is the first "point of impact" where different ratios apply. An industrious race pick might give you a +10% bonus so you could get 66 industry points from your 60 minerals. Or perhaps your government type is prone to corruption so that 20% of your industry production gets wasted along the way leaving you only 48 industry points out of the 60 minerals.
The next step is converting your industry points i.e. production capacity to something useful. To continue the example suppose you are constructing a ship that requires 600 production points to build. If you leave everything as "normal" then your 60 point industry output means that each turn your ship gets built 60 production points and it will be ready in 10 turns. Converting industry points to production points at a ratio of 1:1 is the standard case: 100% factory load.
But what if you are in a hurry? Ok, no problem, now we add the money part to the system. To keep the example simple let's assume converting industry points to production points at 1:1 ratio costs 1 money unit (AU) per point, e.g. in the above case you pay 60 AU per turn for 10 turns (=600AU).
You were in a hurry so you drag the factory load slider to 150%. Now your workers are doing overtime and factory lines go at fast speed netting you 90 production points per turn meaning the ship will be built in little less than 7 turns. But nothing comes for free so you pay extra for the rush job. Let's say we use the square of the factory load so we get a nice smooth function instead of the "step-function" used in MOO3. The price is 1.5 x 1.5 x 60 AU = 135 AU per turn for a grand total of 900 AU.
As a curious after-thought: Setting the factory load slider to 1000% (i.e. 10x overdrive) you'd get the ship built in 1 turn paying for it 6000 AU. Of course, we might want to set a factory load cap somewhere or alternatively working your people to death could have some serious consequences.