A lot of people prefer dedicated streaming devices to the apps in TVs and disc players, because they just tend to work better, but no, you don't "need" the fire TV as well. I use the AppleTV for that, just because it is the best overall, and the most expensive, of course. @Todd Erwin is really...
The TV could be mixing down the audio to two track when going through the optical output, while ARC should be maintaining 5.1. Of course, with a soundbar, the difference can be debatable. I'm definitely not a fan of ARC, but it does serve some purpose, and if it's working, might as well stick...