Great question! It's a philosophically beautiful answer (I think).

Essentially everything in the universe exists in some state which resulted from some finite set of possibilities.

Your height for example is a combination of genetic, nutritional, and possibly behavioral influences. Each of those influences has a different magnitude of effect. Perhaps you personally might have been a few inches taller or shorter if some of those variables had different values. However on the whole people are more or less the same height, with some degree of variation.

That variation is driven by said differences in genes, etc, but there's only so much those variables can differ. They have a finite set of possibilities. So it turns out that most people will be close to some average height, with a predictable degree of variation to either side. A bell curve.

However you need to be sure you have a representative sample of people to determine what that average is and how much variation to expect. If you were to say look at NBA players you might get a very different idea what the average height was, and how much it might vary.

If you were then to try to use that expectation of how tall people are to inform other ideas like the size of cars, and the size of garages, and how much gas people might use, and so on, you could end up with a less than ideal model of the world.

So the best thing to do would be to get a really big sample of people in different situations, so that you can figure out the true average. You want to account for problems with how you selected people to measure (the basketball team) by averaging it out.

That isn't always possible or practical so what we and our brains do is to expect that some variation exists and to simulate this by taking an average, and then adding randomness distributed around that average, because that's how pretty much everything works anyways. We generate a bigger dataset from a smaller one, assuming some degree of randomness which tends towards a central limit.

Randomness is useful in other ways as well.. basically if you want to better estimate the influence basketball has on height, delete that variable at random. Pretend it doesn't exist and run a simulation and get a bettr idea of how the other variables play out.

Statistics is beautiful