Not a user of Wavpack myself, but I think the idea would be to have a sort of "pressure-based fidelity retention" setup: where you'd put the lossy-encoded versions of files onto your portable player device, but as long as your portable device has the space, it would also retain the lossless encodings as supplemental files. As soon as your portable starts running out of space, it treats the lossless encodings as a cache folder, evicting things from it to free up space.

Then, you could actually do mixing in apps like DJay straight on your iPad, on a whim, sometimes—without having planned for it in advance—and make it straight through to mastering and uploading the result... unless you filled up your whole iPad with random videos/apps/etc. in the meantime, in which case the lossless versions would be gone, you'd have to work with the lossy copies, and then, when you get home, restore the lossless copies (or transfer the project to your PC) to master the encoding.

So, not so much when you want the lossless version as canonical (most times when you have a lot of space), but rather when you want the lossy version to be canonical, and the lossless version to be there as a sort of "optional file to enable mastering using this track" file, that's sometimes available and sometimes not.