I'm just in the process of writing a proposal for a new book. I'd say that this is possibly the worst thing that writers have to do (with the possible exception of anything involving accountants - sorry Ian).
In part, the pain is similar to that of putting together a business plan in the real world. It's hard work, yet of itself it isn't a true product. It's just a pale reflection of something that might happen in the future.
There's also the need to constantly suppress the urge to woffle. A good proposal has to be concrete, to interest the reader, even though it's not the real thing. For some reason, the brain is constantly trying to output woffle when writing a proposal (probably to avoid having to do some serious work).
Finally, it's something that triggers a period of agonized waiting. Will they like it? What will they say? It's like doing exams all over again. What joy. But it'll be worth it in the end if the publisher goes for it. And they will go for it. Won't they?...