Region-specific bonus tracks that are literally not available on any streaming service in an official capacity; the only place I can find them are on YouTube - where they could get DMCA’ed at a moment’s notice..
Two tracks off the top of my head like this are:
Don’t Tell Me It’s Over by Blink 182, from Take Off Your Pants & Jacket
I believe that Take Off Your Pants and Jacket was released with 6 different secret songs (in the US anyway). Each album came with 2 of the 6, so you had to see if your friends got the other ones on their albums.
I recently went back to pirating and transferring music to my phone like it was an iPod, lol. VLC is a good player (and works with Android Auto), and Strawberry is good on Linux for listening and library management. The SoulSeek network has a lot of hard to find stuff in flac (I use the Nicotine+ client).
You can customize it to the hell and back. There are even fan made skins that you can apply.
For me, the main draw is its equalizer. You can set from anywhere between 5 bands to 32 bands, along with first and last frequencies. So if you've got different amps and headphones then you can configure it exactly how you want it to be for all the different combinations of setups.