Apple changed everything a while back with iTunes, proving that people would pay for online music and that the future was all Napster-pirated stuff. But services like Spotify pioneered another way to distribute music online using subscription streaming instead of buy-and-download.
Apple was late to join this party, but it looks like the strength of iTunes and the iPhone were enough for it to catch up, and now surpass Spotify. According to Digital Music Trends and Statista Apple is now the biggest service.
Normally, who cares? The only things that matters is which one, if any, you use. But being big in the music industry means special treatment. And that means Apple Music gets more exclusives, special releases, extra content, and so on. They can also negotiate prices better, which hopefully means the cost of Apple Music won’t go up any time soon.