This is an easy, bottom line and great thread (up to now). And for someone like me having possibility to check and get almost daily necessary security (main OS software at the first place) and stable apps upgrades is something that makes main difference to me.
In short, I’d qualify this as main advantage of any real Linux phone (without need to quote some/any official source). Maintaining PureOS as stable as this gets might easily be one of the most important things out there for myself (trusting in great job they do, without ignoring great jobs and efforts from other Linux communities). After this, in addition, having Librem 5 being free of blobs, having capability of making phone calls, turning on and off few HKSs, etc., etc, etc. can easily, IMHO, make someone proud that such product exists.
To manually upgrade packages through the CLI:
~$ sudo apt update
~$ sudo apt upgrade
To update all your installed applications and runtimes to the latest version, run:
~$ flatpak update