It's also a surefire way to stall an idea out in paper form that by the time it comes to implementation, people have gotten distracted by other urgent work and the idea never comes to fruition.

Since time can only be spent once, you are essentially picking between a quick&dirty prototype and a nice document.

The document invites a bikeshedding session and consumes reviewer time, the prototype often allows you to immediately see whether the idea is worth pursuing, it may directly deliver a part of the benefit of the completed solution, and most importantly, it'll show the actual weaknesses in the design and the real constraints you have to deal with in a way that a theoretical review can't.