It sounds complex, but it's actually quite straightforward, and even card game newbies will pick it up after a brief tutorial
Its most common use is to copy a simple, cheap spell, like a 1-mana cantrip or a light removal spell
For example, if your opponent knows what the top card of their library is and you cast Siphon Insight, you don't have to tell them where you put that card
As for Humilitys impact on the game
Instead of 24 packs with 6 cards each, a premium booster box contains 10 packs of 10 cards 100 cards total per box
So to recap you draw 7 cards, each of you chooses an active Pokemon, you each place six prize cards, you then resolve all mulligans, and then bench any other basic Pokemon onto the "bench" that you wish