As Magic the Gathering is a game not only played by adults, but many teenagers as well
However, you can consider playing fewer than four copies of a card if: It gets worse in multiples (such as legendary creatures) It's only useful against certain matchups It's a win condition, and your control deck will have time to dig through the deck and find it You're stuck choosing between two cards which have equally strong, similar, but slightly different effects (you could play two of each) It's a slightly worse version of a card you have four copies of already, and you're including it because you want even more of that effect
It's the commander you just can't get rid of
The book side is sometimes relevant, especially when converting to your janky infinite mana and storm wins via Hullbreaker Horror and Jadzi, Oracle of Arcaivos, but in my experience, youre more interested in bouncing the book back to your hand than actually using it
If tails discard a energy attached to this Pokmon