Sodium citrate for example helps your cheese melt creamily.
The powder likely contains the right balance of additions that give you a great creamy mac & cheese.
A good reference: https://www.seriouseats.com/recipes/2016/11/sodium-citrate-b...
Another Serious Eats reference. https://aht.seriouseats.com/2011/07/the-burger-lab-how-to-ma...
Eh, is this a US thing? Here in the UK, I just use cheddar - any brand, any maturity you fancy, and it melts just fine.
I spent several months in the US around 2005, and I do recall the cheese was pretty terrible - bland, with an odd, plasticy texture.
Aw, dude, you're supposed to remove the individual plastic wrap from the cheese before serving!
In Britain they label it "singles" or "slices" or something equally meaningless.
They do seem able to use "cheese" when English isn't a national language, though they then can't use the local word for cheese.
They are called "pasteurized process cheese food" or "pasteurized prepared cheese product" depending on their ingredients. There are pretty specific FDA regulations on cheese naming that are presumably similar to the equivalents in the EU: https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFR...