Depending on the TV you are using and the streaming services you are using, you may have better results with the Firestick for streaming applications over the built-in apps on your TV. This is dependent on the TV you are using and how seriously they have taken their own take on streaming services.
Audio can be better from some services using an external device due to the limitations of digital audio from the TV to the receiver.
For me, it's about the better usability and consistent remote that we get with our Roku devices vs. the built-in features and what others offer.
Amazon Echo is mostly about the voice control. Not sure if it's integrated to turn on the receiver or do other things in your home, but the Echo is perhaps not the best audio you may get, but it may be the most convenient way for you to listen to things and control things.