Why is that? We continue with Siman Kuf Peh (section 180). He brings a few sources together: the Zohar, a Gemara in Berakhot, a Gemara in Sanhedrin, and some Rishonim (early authorities). The Shulchan Aruch does not put them together; they are two different Halachot (halachic rulings), but I think they have the same spirit.
He says that in Halachah, we don't remove the bread from the table until after Birkat Hamazon. In Halachah Bet (second), he says that somebody who doesn't leave a piece of bread on his table doesn't see Beracha ever. So don't eat it all; leave a little piece. Now, it's not exactly the same Halachah, though obviously connected. What's the source? What's the reason? On the first one, you don't remove the bread. It's famously quoted because... So, you're saying that for Birkat Hamazon, you have a Heker (reminder). You see what it is about. To be honest, everybody knows you're doing Birkat Hamazon because you have a meal. Nobody's saying Birkat Hamazon in the air. It's not that you need a piece of bread to remind you that you're saying Birkat Hamazon for the bread? Come on. You fool like this. What are you talking about? The words, everybody's together. You need a reminder for that? It's a little surprising.
The Mishnah Berurah continues and brings this: “Also, the Beracha doesn't go on something empty.” Same question. I'm not planning my next meal right now. I'm saying thank you for what is not here anymore. Why do I need to have a piece of bread? Because the Beracha doesn't go on something empty. I'm not looking for the next meal. You're not making plans for the next meal. You're saying thank you for the previous meal, so obviously there's nothing left. I would say, the less that's left, the better. If you have all leftovers, what do they tell you? “I didn't like the food,” huh? So, what's going on here?
First of all, we can see that maybe the definition of Beracha is different from what we think. That's why, in the Amidah and in Birkat Hamazon, there's a construction: you thank Hashem for what happened, and you are planning your next meal, by the way. Part of Birkat Hamazon is that you're planning the next meal. Beracha is a conduit. It's a flow. It's not a thing. “Oh, I got a check. Oh, I'm happy.” No, that's not Beracha. Beracha is financial planning. You have to have a flow. You have to plan. That's Beracha. You got a good check? Happy. No, that's not how it goes. You have to have a Beracha. There's a flow from before to after. That's the Beracha. That's the Beracha. That's the Lashon (wording) Beracha: Al Ha'avar ve'al Ha'atid (for the past and the future). You see how in the Amidah, you have Hoda'ah (thanksgiving) and Bakasha (request). Here too, you have Hoda'ah and Bakasha. That's what it means. You have to have something to continue because it doesn't stop there. If a Beracha stops somewhere, it's not Beracha. That's an end. Beracha, Barech, is Shefa. A Shefa doesn't end. It's like a river that continues flowing. If you get to the lake and the lake is the end, that's not Beracha. Beracha is a continuity. That's the Lashon Beracha.
And therefore, you have to have something. In order to continue that Beracha, they bring a Pasuk (verse): Yahiv Chochmata (intelligence) le-Chakimin. HaKadosh Baruch Hu gives Chochmah to the smart ones. You need to have intelligence to pull intelligence, to continue intelligence. So that way, Beracha is continuous. That's why it says here that you have to have a piece of bread in order to be Moshech de Beracha (draw down the blessing). It's important to understand how Beracha works. We all want it, so you've got to understand how it works.
The other thing that's important, which the Mishnah Berurah quotes, is that the reason we leave a piece of bread is Kedei Sheyehei Mezuman Le'Ani (poor person) Sheyavo. It's because part of the Beracha is also being able to share it with people. If you can't share what you have with others, it's Kelalah (curse), not Beracha. So if you show that you don't finish it all and you keep something, at least theoretically—these days it's a little different. People don't knock on the door for a piece of bread, but at least you show that there's a piece of something: Kedei Lehash'ir Le'Ani Sheyavo. In order to have an ani who could be taking part in this, I think it's another aspect of the Beracha. The Beracha comes if you could share with somebody else. And therefore, the source says that you say Birkat Hamazon on something that is left on your table.