What is the difference between WAV and FLAC?

You might be confused about different concepts:

• FLAC is a lossless audio codec (its container also happens to be called FLAC, but the main idea here is the actual codec).
• WAV, on the other hand, as a container can hold numerous kinds of audio codecs, but mostly, you'll find PCM-encoded audio.¹

Since FLAC is all about mathematically lossless compression, FLAC files will be smaller than corresponding PCM-encoded WAV files, since PCM doesn't allow for lossless compression and just represents the data as-is.²

So, simply put: Take a WAV file with PCM-encoded audio, and the corresponding (mathematically equal) FLAC file will be a tad smaller. The downside is that FLAC is not as widely supported as WAV. For example, most (all?) operating systems won't play or convert FLAC files without extra software.

Bottom line, I'd say their practice is redundant but a nice gesture for folks who don't want to download massive amounts of data and prefer the compressed version.

_{1 – For example, variants of MP3 can also be stored in a .wav file.
2 – The analogy for images would be that BMP files contain the raw image data in an uncompressed form, whereas a (lossless) PNG is much smaller, but showing the exact same contents.}

FLAC is a compressed lossless audio stream, WAV is uncompressed lossless audio stream. FLAC is like ZIP in audio world, you can compress/uncompress the data multiple times without any loss. But compression ratio is better than ZIP, because this compression format is tailored specifically for audio.