I get that people cheat by dealing off the bottom of the deck, but how does this actually advantage you / your partner? Does it mean you have to see what’s on the bottom of the deck for every card dealt, which sounds very difficult, and then know if that card will advantage you/partner, which sounds generally low probability?

For example there is a scene in the movie Rounders where Edward Norton deals something like three 7’s and two A’s to Matt Damon. Doesn’t this require, at best, seeing every card on the bottom, and getting very lucky that all of those cards happened to turn up on the bottom? Also no one is supposed to notice you analyzing the bottom of the deck constantly? (He does get caught but for a different reason.)

I feel like I’m missing something because this seems impossible to pull off and very unlikely to yield results except over a very long period of time.