They don't have everything. I have plenty of tracks on my drive at home that aren't available. ALSO, sometimes you'll find a track you like and save it locally in Spotify, then Spotify decides they don't like that track anymore and you no longer have access to it. It still shows up in your library but it's grayed out.
Also their shuffle button is hot garbage, at least on Android. It's been garbage for years and it recently got even worse.