What's appropriate for my display, exactly?
You have to base it on the VIEWPORT, but that's VARIABLE because the USER can change that shit.
Any viewport change and you risk having to download the newly "appropriate" version.
The whole premise of why we'd want to do this is retarded as well. Phones are getting resolutions of 2560x1440 now.
There's more to it than that, and yet more coming in the future. Yes, media queries tend to be primarily viewport queries. Viewport data is more complex than just pixel dimensions though, because a browser pixel is not a device pixel. This is why device-pixel-ratios are also supported. A 2560x1440 phone likely responds to a media query as ~854x480@3x (the math isn't right, I wonder what the real device pixel viewport size is).
Picture/source also supports mime-type alternation, just like video/audio sources do. This allows content to be delivered in preferred media types (e.g. webm, webp) where possible with fallbacks to less-preferred types (e.g. h264, png/jpg/gif), potentially reducing bandwidth and cost.
The same group that led the picture element is now leading element queries, which will allow size-based queries to be derived from the size displayed on screen, rather than the size of the viewport itself (as in, placing a responsive image in a sidebar will have different download characteristics from placing it in a full-width column).
And browser vendors can develop selection algorithms based on user preference (e.g. prefer faster downloads) and network conditions (e.g. high latency cell, bandwidth limits, etc) rather than viewport conditions alone.