>>9987079
Here's a quick rundown:
You and your opponent are each dealt 5 random cards.
The rarer your combination of cards, the more "valuable" it is.
Let's say your opponent was dealt a 5,6,7,8 and 9 of the same suit (straight flush.)
Immediately, he's going to think he's won, because the odds of you having a better hand are extremely low.
Now, let's say you got dealt a A,K,Q,J, and 10 of the same suit (royal flush.)
You have the single most valuable possible hand in poker, and are guaranteed to win.
But your opponent is operating under the assumption that the odds are so tremendously in his favor that he's already won.
You want to make him think he's winning so he'll throw more of his money into the pot, thinking that he's baiting YOU into throwing more money into the pot.
At a certain point, the betting stops, and he shows his hand. Straight flush.
You show your hand. Royal flush.
You just took everything from him.
Why would you show your hand sooner than it was necessary? (Rhetorical question, but maybe I'm missing something!)