Why does Unity need to recompress ogg vorbis audio files?

When I import an ogg audio file, unity re-compresses it.... into an ogg audio file.

Why is that needed? Or is it really compressing them into a different format?

If not, can I avoid that step somehow?

when you drag an audio file ogg/wave or ... you can adjust the settings of the imported audio in the inspector. i think the reason is import settings of your audio file are different from your ogg file's properties, so unity needs to change some properties like compression level or bitrate or ... to make these changes unity should recompress your file. i think if your file and it's import settings has no difference there is no reason for this and unity will not recompress it.