I don't understand purpose of airplay

It lets you stream content from your computer or iOS device to Apple TV.

tt2

Use AirPlay to wirelessly stream content from your iPhone, iPad, or iPod touch - Apple Support

https://www.apple.com/support/appletv/airplay/

With the Apple TV in most cases you can watch videos, listen to music, or view photos directly from the ATV using Apple's various services or view content from multiple providers using their ATV apps (like HBO Go or Netflix). AirPlay is not needed.

In some cases the ATV app does not exist (like Amazon Prime Video) so videos or music available via Safari or non-ATV apps which may be playable on your computer or iOS device can be viewed on your TV by using Airplay to send the output of the device to the ATV. It does not work in all cases because of restrictions the content providers may put on using Airplay.

Also there may be cases where you may wish to view your computer or iOS device on a larger screen to share whatever you are doing with others. Airplay mirror allows you to accomplish this.

AirPlay lets you stream content (video, music, etc) from a iPhone, iPad, or Mac directly to the AppleTV. It also allows you to stream from the AppleTv to certain speakers and devices wirelessly as well. It's pretty useful - I find myself browsing Facebook, etc, and using airplay to send a video to my TV instead of watching on my phone. It works very reliably and very quickly.

For the most part I find it a gimmick I rarely use, but once in a while it's great to show some recent Photos to mother-in-law when she visits.

It's pretty reliant on decent connectivity and undoubtedly works better with newer iOS devices.

Undoubtedly clever, just not perfect.

I used it quite a bit with BBC iPlayer from my iPhone to the TV before they released a tvOS version for Apple TV 4.

tt2

We use airplay to stream Amazon Prime TV shows and movies from an IPad to my TV set through an Apple TV box. Great audio and video quality.

Really?

I may need to revisit that Amazon app as the quality was tryly dire when it first came out.

That said it's simpler to view it on my Fire TV.

There are some shows not on AppleTV, like anything from the Sci Fi channel, but they do have an iPad app. Using AirPlay we can watch SciFi Channel shows on the TV without having to physically connect it to the TV.