HLS offers the possibility to encrypt the transfered video data. Is it really needed to use that encryption if I'm running the web server over HTTPS? By using HTTPS the data stream is already encrypted, so why would I use an extra layer?
It serves a totally different purpose. SSL is terminated (decrypted) by the browser/http client. Encrypted video is decrypted by the player/app. Basically, this allows for putting the video itself on a server accessible to anyone (with or without TLS) but unless you have the decryption key, the files are useless.
TLS is for your benefit (privacy), encryption is for the video owners benefit (control of decryption key)