Lets first explain the meaning of HLS
HLS stands for HTTP Live Streaming and is Apple’s proprietary streaming format based on MPEG2-TS. It’s popular since it provides the only way to deliver advanced streaming to iOS devices. It often mistakenly gets defined as HTML5 streaming, but is not part of HTML5.
So what is HLS good for? Why not just use mp4?
Visualise someone riding around San Francisco in a bus. As the bus drives around, they move from WiFi, to 3G to 4G LTE to Edge (anyone remember that? it’s slow). Their internet connection changes as they move about. HLS allows the bit rate quality of the stream to rapidly adjust to the user’s bandwidth.