I used to hate trying to sum up my work in a single, succinct phrase or sentence, because I felt like trying to compress it into such a space only did violence to it. I now know this is a silly attitude to take, because being able to sum up your own stories succinctly means you understand what they are at heart really about.
Some of this came back to mind when I tried to give someone else a succinct summation of Shunga-Satori, the book I'm currently working on, and the phrase "a Cronenberg-esque fairy tale" popped out. To be honest, the first thing I groped for was closer to David Lynch's summation of Eraserhead: "A Dream [sic] of dark and troubling things." But I liked the contrasts evoked by what I settled on: the body horror (and body fabulism) of Cronenberg vs. the flavor and atmosphere of a fairy tale. Although, as anyone who has done even a cursory reading of the uncensored Brothers Grimm will know, fairy tales by themselves can be quite Cronenbergian. But you get the idea.
A head-of-a-pin summary doesn't just help you sell the idea to others, or write good flap copy for the book (although those things are a sizable help). They help you keep the work focused on its intentions. When I landed those two keywords, Cronenbergian and fairy tale, they helped me either strip out of the book anything that did not fit its proper vision, or take what was still there and re-focus them to fit. That said, they were not so much containers as they were meta-tags, so to speak. They didn't forcibly exclude everything that didn't 100% match those terms, but they described the vast majority of what the book was supposed to be about and how.
Elevator pitches are also a fun brainstorming process. My friend Steve and I often throw "what if X but Y" type formulations at each other just to see what makes each others' gears turn. None of them are binding (okay, I lost a bet with Charisma), but every now and then they serve as a spark to a new story, or the clarification of an existing one. Originally, though, I didn't think of them as tools, but as something one came up with while holding one's nose: a necessary evil for the sake of making a story comprehensible to people in the short window of time you have their attention. That's one of the reasons you come up with them, but far from the only one, and not even the most important.