Uncompressed pieces of audio are immensely large, so large in fact that they can't be stored easily and shared without being compressed. Compressing a song means estimating or approximating parts of the song's sonic information to make the audio smaller.
The amount of information that gets lost in the compression process helps characterize the differences between audio file types. HD WAV's are compressed the least, meaning they are the largest file size but also are the highest quality. These are used by musicians who want as much of that original sonic information retained as possible, and are willing to make the files much larger to do so.
HD Wav's offer downloads at 24 bit with sample rates higher than 44.1Khz, and can go up to 96k. Essentially the HD Wav will download at whatever sample rate you uploaded the file at, up to 96 Khz.
Uncompressed WAV's are the second highest quality besides HD WAVs, but are also smaller in size. Uncompressed Wavs are limited to 44.1 Khz 16 bit, which is an industry standard for CD's and other audio formats. These formats are commonly used to upload to streaming services, which then compress the file further to an MP3 to allow them to store millions of songs.
MP3 files are the most compressed, and are significantly smaller than Uncompressed or HD WAVs. These files are commonly used because their small size allows lots of songs to take up a small amount of storage.