What do these additions to common spice mixes do with your dish?
Salt: 

Added as a preservative and taste maker. And usually indeed lots of it. 
Very broad and coarsely put, salty is usually perceived as tasty.

Corn / Potato starch: 

Added as a thickening agent. They also thicken sauces while keeping them more translucent than regular flour does, although I'm not sure if that's the reason they use those starches over flour, but I think it might.

As GdD mentioned, starch is also added as an anti caking agent: It absorbs moist to prevent lumps from forming.

Dextrose / Sugar:

Not sure, but I would guess mostly for taste, and partly to balance out any acidic ingredients. Again, very broad, and very coarsely put, humans like sweet. Sugar equals tasty.