A really common piece of advice given to scholars when talking about their research (whether in a book proposal or other context) is to answer the "so what?" question. I kind of hate that formulation and here's why:

So many of the scholars I work with are writing abt the history & experiences of real people. Often they're people from communities that have been marginalized bc of racism, nationalism, etc. To read that scholar's book proposal and say "so what?" would be, IMO, deeply insulting

I try to force myself to find another way to get at the issue. It's not "so what?" because of course the information this scholar has uncovered and synthesized is important and matters to them and to a lot of other people too.

What I might really be trying to ask is "who is this important to and can you tell me *why* it's important to them?" Or, "how could this knowledge you've generated make a difference to the people you're writing for?"

I don't ever want my questions to imply that the work isn't important or won't make a difference. I just need the scholar to help me—a person who doesn't know what this scholar knows— understand what is really driving this work and why they think it needs to exist in the world