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Is this basically a simpler way to do "Data|From Web" or "Data|From Other Sources" where the burden is on the developer instead of the user?

Yes, that's one way to look at it. In practice most people send CSV's now, so it's a simpler, streamlined way to do that.

A big advantage of Voyager over both of those options is the ability to send custom data per-user without building an ad-hoc web UI for it.

Right, per-user is a very useful feature. Given that 99% of the world's real work is done in Excel I think this could be a good product.