I pay for Spotify for 7 years or so now and i'm so sick of all the ads. After every update there seems to be an: oops, sorry, you have ads now. Podcasts are filled with ads. I was just listening to a podcast where they shoved in 3 ads mid-sentence. How long until musicians put ads in their songs. I'm just so sick of it.
Ok so if you connected your Spotify account, then it should have your saved playlists and so forth in your library. You can stream or even download these tracks, just like with Spotify Premium. You should also be able to create new playlists/save content to your library.
It even has some features that Spotify doesn't have. For example, you can totally block specific artists. So like for me, I fucking hate The Dwarves, and Spotify always recommends them to me. On Spotube, I was able to block them. (I'd have to refresh my memory on how.)
The catch is, it doesn't use actual content from Spotify at all. Instead, it grabs these tracks from YouTube's API. At least, that's how it worked when I used it. If you try to download too much content at once, it will make you confirm that you understand that your ISP or someone might notice and take action, so ... download judiciously, I guess?
I usually have a pretty solid mobile data connection, so I never bothered with downloading.