Velveeta was developed in 1918 as a way to use the byproduct of cheesemaking, whey, and various cheeses (because it combined several cheeses). I don't know how much the formula has changed (probably quite a bit), but it was always marketed as a cheaper alternative to cheese.
It was sold to the Kraft Company in 1928, but I'm not sure how popular it was before the 1950's.
I think it was possibly Alton Brown, I saw suggesting that "melting cheese for dummies" (my words I think, not his) start with Velveeta. So start with a little Velveeta and add "real" cheese to it for mac and cheese, fondu, etc...
I've never bought Velveeta regularly, but I have used it once in a while (Velveeta and Pace Picante is my version of the puke dip).
With my salt taste changing, I'm betting it's far too salty for me now (it was already too salty for me the last time I had it several years ago).

