Gacha games usually employ an index of items to check, and a Pseudo-Random Number Generator, with either your User ID as the seed or in more extreme cases, the server time as the seed. However, Gacha RNG has a "drop rate" or a probability that you would get your preferred item in a N number of drops. And of course, that involves a lot of money to thoroughly test this, so why not use our knowledge of data science to figure out that instead of wasting buckets of money just to figure out the distribution by hand?