Was working yesterday ( I work part-time at a card shop by the way) and saw some people buying an insane amount of booster packs, all in the aim of pulling the foils that they desire. That set me thinking on the topic of why different rarities were created in the first place.
Rarities don't change the way of playing the card game. If a card is good, you will still run 3 copies of it (or 4 copies if you are playing cardfight vanguard or pokemon) regardless of whether the card is a foil or not.
Rarities don't necessarily have to be the defining factor in the pull ratio of a certain card. If a company wanted to make the pull ratio of a card lower, they can just make more cards into normal rares and make necessary adjustments to the pull ratio of the normal rares per box while maintaining all cards to be non-foil.
Rarities screws up the younger players (true story). Imagine a shop opening a new box, and a experienced player who knows how to feel out the foils come into the shop at the precise moment. A couple of mintues later, the box will have a few packs lesser which are bought by the experienced player. The box will still be very full, and when some younger players come in and see it, they will happily start buying the packs without knowing that the foils are gone. This leads to them never having a chance at getting the foils that they need to better their decks, and when this kind of situation arises, they always quit the game (don't talk about them having the alternative of buying whole boxes, not everyone is that rich)
However, to balance out the argument, rarities are much needed in card games.
Rarities are essential in the survival of the card game. Many people who buy boxes and boxes of cards are collectors. They covet the foils, and when a card game ceases to have foils, they woulden't have anymore interest in purchasing the cards because all the cards looks so plain and similar to them now. Therefore, they will stop buying and sales will decline.
Rarities helps to prevent several scams (to a certain extent). For a long time now, rarities are used by the less experienced players to determine card prices. Therefore, if rarities are removed from the card game, experienced players can simply offer an extremely low price for a good card (or ask for free cards) and the less experienced players will agree to it as they don't have a guage to use for the card prices. However, when the card they possess is a foil, they will be more careful of the price that they are letting go of it.
Thinking back, I honestly can't recall what was the first card game to be released, but I'm guessing that rarities already existed since the first game and other card games that were released later had no choice but to follow the usage of foils so as to attract/retain their player pool.
Comparatively, I actually find Pokemon to be a fairer card game as all their non-foil cards have a foil equivalent (except the better cards that only have a foil version) and all their booster packs have 2 foils in them. Sure enough, the pokemon ex and full art cards still can be felt out, but with foils in every packs, it would attract the interest of more players and give them hope when they are buying packs regardless of what they pull.
With a foil version of every card, collectors will also be more interested and buy more to complete the entire set. Remember a time when Yugioh TCG used to have an ultimate rare version of the card for the rares? (i.e ulty overload fusion, ulty neo-spacian grand mole and ulty future fusion). People would buy boxes and boxes just to get these ultimate rare versions.
In my opinion, rarities is the determining factor for the sales of the game. Correct decision of which cards to push to a higher rarity leads to better sales. Some minority will be sacrificed in this process but a large majority will still be drawn to the allure of these shiny cardboard.