Ultimately, music discovery is a means to connect us to music. Your piece doesn’t address the fundamental ‘why’ of music, and without addressing that it’s hard to draw any fundamental conclusions about music discovery. I’d argue that the fundamental ‘why’ of music discovery is almost the same as the fundamental ‘why’ of music, and all art.
So taking the ‘why’ of music as a question that doesn’t need answering, I suppose the other part of the question is why we need music discovery services. The internet empowers content creation and sharing; with evermore content, the work of filtering through to find what is worthy gets shifted onto consumers. The music industry are no longer the ultimate gatekeepers. The burden of determining what’s good is on the consumer now, and music discovery services alleviate this by making it easier for us consumers to be our own gatekeepers.