Do you not use your Echo for prime music?
About Alexa and Prime, I would also point you to this from the horses mouth: https://www.amazon.com/primeinsider/tips/alexa-tips.html .
I'm curious by the way: what do you use the echo for which your phone doesn't do better?
The echo serves as a good home music speaker which I can control through spotify on my phone/computer/voice (through echo).
Outside of that, setting timers, asking the weather, random questions (my girlfriend uses cooking/math conversions a lot, but alexa isn't as good as google for random facts from my experience). My girlfriend loves to use it while getting ready and cooking since her hands are busy for hours during those times.
Alexa is easier than pulling out your phone, finding the app you need, waiting for it to load, possibly having to authenticate, then making the change you need, in lots of cases.
Thermostat, lights, podcasts, quick questions, timers and more.