I think they sort of depend on our ignorance (as in, not knowing any better). Like you HAVEEEE to buy this brush or the blush won't work. You can't wear concealer without foundation! You can't wear lipstick without a liner and definately you can't apply it without buying our sharpener and lip brush!
I bought a MAC spray on foundation, wasn't crazy about it, then was told thats bc I had the wrong brush. So I went back, bought the same thing and the brush. $75$ later, still don't like it!