When I have the inclination, I go through some of that collection and choose songs to add to the playlist in my car. This was a necessity as, until fairly recently, I had more music than I could fit on my phone (and still have pictures and apps, etc. #firstworldproblems). This gave me about 300 songs that I knew I enjoyed or that I knew my children enjoyed that we could listen to on our trips.
Realizing that I could actually hold all of the music I own in the palm of my hand, I dumped it all from my aged laptop onto my iPhone. This has greatly confused my car. I've happily used Bluetooth to play music from my phone through my car's speakers. In the past, Mera would connect with my phone and play whatever music I had last played. Unfortunately, a recent update has made it so that only happens some of the time, while the rest of the time it plays music from the entire selection.
In short, I've been hearing songs that I hadn't heard for years. Occasionally, I've found ones that I then add to my "Car" playlist or even my more upbeat "Exercise" playlist. Sadly, it turns out that I don't care for many of the songs that are on my phone. I'm not quite willing to delete them, but I have been finding myself frequently saying, "Hey, Siri. Please shuffle car playlist."