One ques - does the basic Plex server serves the purpose for movie/audio streaming or getting Plex Pass subscription is needed?
Depends on how much into the Plex ecosystem you want to get in:
1.
Hardware transcoding as
@OM_2K19 already mentioned. If you have a recent Nvidia GPU, this will allow you many streams at once, especially if you're transcoding a 4k HDR movie/TV show. Otherwise, it will be done on your CPU via software transcode. That's fine if you have an 8+ core modern CPU, but if you're using a NAS with a Celeron inside, hardware transcoding is the only way to go as it will use quicksync on Intel systems.
2.
Dashboard: You'll be able to monitor your server from your phone or other devices with the Dash app. This may or may not be useful for you depending on how many folks use it.
3.
Android/iOS app support: The free version doesn't allow you to view stuff on android and iOS smartphones. If you buy Plex Pass or buy the phone only plan, that will be unlocked.
3.
Downloads: You'll be able to download the show for viewing later on your phone. I use it when I go on trips to watch my backlog.
4.
Skip intro: Automatically parses the file and has a Netflix/Prime-esque feature to skip the intro of any show.
5.
PlexAmp: App for streaming your own high-res music collection. Supports every format you throw at it and it will bitstream to any capable client device.
6.
HDR tone mapping: Will tone-map your HDR content to view on older devices with SDR only support.
There are also some other features like advanced sharing, trailers, DVR etc but I've not used them much. May be useful for you.
I bought the lifetime Plex Pass last year for about equivalent to $80. Definitely worth it if you do or plan to have a sizable self-media collection. And it's not just pirated content either, I also share my photos and videos from functions/trips I've attended in past over Plex.