You're misunderstanding the currency cost and the purpose of exotic focusing.
The green numbers are what you have in your inventory, meaning you currently have two exotic engrams. The white number is the cost, which is 1 exotic engram.
The Witch Queen engram you are hovering over does narrow down the pool of exotics you can get. You can also tab over to another page on the focusing screen to pick the exact exotic you want to focus that you have obtained before.
The reason you wouldn't just leave it up completely to RNG is because you would be randomly getting an exotic from all five years of Destiny as opposed to focusing a Witch Queen engram with 5 exotics.
Did that make sense?
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Yes but you can't get any that you have not already unlocked, so why bother?
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You bother for better rolls on your exotics for your builds. You still have to work for the new exotics by doing the daily Lost Sector or Vex Incursion
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Its for people who want a specific exotic with high stats. Instead of gambling on both the exotic and its stats you only have to gamble on the stats. It just makes farming specific exotics easier since you dont have to worry as much about getting the wrong exotic. If you arent targeting an exotic or specific stats than there isn't a reason you would spend the matetials to focus it.