I don't think I've ever bought keys for any games that have them but I do spend a fair amount each year on the Dota 2 International Battle Pass in the hopes of unlocking some of the rarer items that come with it.

To further elaborate the battle pass works on a level system and you can pay real money to level it up (there are ways to level it up which doesn't involve paying a cent but paying is the quickest & easiest way to gain levels). Now every couple of levels you get one of three different treasures which are very similar to crates/ chests in other games only you don't need a key to open them, instead each treasure contains six or so unique items called Immortals—the second highest quality level for cosmetic items in Dota 2—and you will not receive a duplicate Immortal until you have received at least one of each from that particular treasure. In addition to the default Immortals there is also a chance for a treasure to contain a rare, very rare and ultra rare Immortal item.

In case you have not realised it yet, it can be very expensive to get ultra rare items in Dota 2 because of how many levels you have to purchase before you receive another treasure and thus another chance at receiving a rare item. There's also a physical reward you get if you level your battle pass up to level 1000 and I did it once when it was first introduced but I was put off from ever doing it again because after spending over R5k to reach level 1000 I did not receive the ultra rare item I really wanted because of Valve's chance system.

This year however my luck seemed to increase and I'm not sure if it's to do with China forcing Valve to disclose drop-rates or whether those drop-rates even apply outside of China, but I actually received more rares this year than any other year previously. I even received a few duplicate rares as well despite only levelling up my battle pass to level 500! But in the same breath despite levelling it so high I still did not get every rare which is what always makes me regret spending money on The International.