These days we want to bundle requirements into releases. (Large proejcts are highly risky and breaking things down reduces some of this risk.)
As the requirements manager you are the one who has the vision for what will be in - and out - of the release.
It's a useful excercise to be able to wrap up your release into an elevator pitch. It helps you focus.
And face it. If you were seeking VC money, you'd have to be able to render your complicated vision down to a few snappy phrases.
Here's a website to help you think through or elevator pitch. (And here is the inspiration for this post.)