What do these additions to common spice mixes do with your dish? Where I live, supermarkets sell all sorts of "spice mixes" for specific dishes, like "taco mix" or "spaghetti mix". I used to buy these, but then at some point started experimenting myself and stopped buying them.

Occasionally when I'm lazy, I still use these mixes to prepare food. But on reading the packet ingredients, I notice that most of the spice mix seems to be made up of things that I normally never add to my food, but I don't taste much difference for it.

Why are these ingredients added to the mix, and what are the effects on the flavor, taste and/or texture of the dish?

The ingredients include:


A considerable amount of salt (at least 5 or 10 times what I would put in myself)
Dextrose or other forms of sugar
corn- or potatostarch
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.