Managing a hospitality operation is a constant behind-the-scenes jigsaw puzzle rearrangement. As professionals, we cannot expect guests to fully realize what is going on to make their time with you actually work.
The title phrase of this post is my favorite go-to and something I empower all staff to say in response when we're unable to accommodate a guest request. It works for situations big and small. Perhaps a walk-in guest asks the host for a table and the host quotes an accurate wait time of 30 minutes. The guest sees an empty table and asks "why can't I have that one?" The host kindly lets them know that is held for reservation coming in shortly. Now whether or not the guest starts a back and forth, a good way to to proactively put a period on the conversation is to say, "I'm afraid that table is for a reservation coming in shortly, thank you for understanding, but we'll look forward to having you seated in about half an hour."
There are countless times to use this phrase. But it fulfills two key motives at one time—you thank the guest (deference and politeness) and imply they understand (which most will accept as the compliment and adds finality to the conversation).
So when you use this phrase, first remember that the guest likely does not actually understand the behind the scenes jigsaw puzzle going on, which is not their fault. Second, be polite, empathetic and then thank them for their understanding of their situation.
P.S. This is not to say we shouldn't always do everything we can to accommodate a guest request! Just a helpful phrase when you're doing things the right way and need an assist to communicate.