Every action you take at the table should either result in you winning the hand, or obtaining information that will help you make the correct decision going forward.
The best example of the value of this rule I've seen this year came from the WPT Legends of Poker tournament.
Frankie O'Dell and Joe Pelton are playing heads-up for the title. They start out with reasonably close chip stacks - Frankie having perhaps a 5:3 advantage. Blinds are 40K and 80K, antes are 10K - meaning that they are playing with stacks well in excess of 30 times the big blind. Frankie is delt J4h, Joe has Q8o. Frankie calls from the button and Joe checks.
The flop comes QQ4 with one heart and two diamonds. Joe leads out with a 100K bet into a 180K pot.
Now Joe has been doing that sort of thing a lot, whether he hits or not. So that bet doesn't really help Frankie any. Frankie, of course, has two pair on a paired board - he overcame 20:1 odds to get that far. The odds of his opponent having hit trips with unpaired cards is about 73:1. So what does Frankie do? He raises, of course. Why? Because if he just calls, what will he do if his opponent makes a big bet on the turn, which will likely be an overcard to the 4? If he raises here, he can either drive away a bluff hand like, say, JT or A9, or perhaps even drive away a pocket overpair. But if Joe calls a raise or goes over the top, he can know that his fours are probably no good. Frankie makes it 350K to go.
This can only tell Joe he's got it. The only hand that has him beat right now that could have and make that raise with is a Q4 - and if he has that, well, that's just amazingly bad luck. If Frankie had a big queen, you'd think that he might have raised before the flop, and if he had a queen and a 9 or ten you'd think he'd just call. So if I'm in Joe's shoes, I'm going to do whatever it takes to get all the money in at this point. The question is, how? Just call here and bet again on the turn or raise? If it were me, I would just call and try and make a big bet on the turn hoping that Frankie might hit two pair or something. My guess is that Joe was afraid of the flush draw. He decided to raise it, making it 750K to go.
So if you're in Frankie's shoes, what might your opponent have that would allow him to respond so aggressively? Either a queen or maybe a bigger pocket pair than 4s. If he had either of those, don't you think he'd be sure to throw down in this spot? Wouldn't you know that your fours are no good?
Instead, Frankie went all-in, Joe instantly called and Frankie did not get the runner-runner flush or quads.
Joe took a 3:1 chip-lead with that hand. Joe just took that lead and ran over Frankie to win it. And you just have to look back at that pivotal hand to see exactly why.