Well, at least in Plex's case I'd assume it's to generate a reoccurring revenue stream to continue development. I question how sustainable the "Plex Pass" feature is with so many people (at least that I know) who bought a lifetime subscription ages ago. I personally bought a Plex Pass for like $49.99 around 6 or 7 years ago and hasn't given them a cent since then. I support their efforts like Tidal and ad-based TV and Movies if it means I get new features without having to move to a monthly or yearly based subscription service.