So I'd spin the question around: why NOT offer a desktop app option—considering that electron makes it a pretty light investment for any team already building a web application?

If you already need your product to work in the browser, you're a small-ish team, and your feature set is in motion, I think it's a good strategy to do an electron app because it further incorporates your product into the day-to-day experience of your users. There will certainly be a point where offering a native desktop app becomes worth the investment, but that may not be day zero.

Meanwhile, would love to see google work more closely with the electron project to help (1) use the chrome runtime if it's already present and (2) at least allow two electron apps to share a runtime footprint.