It’s open source, it’s easy to setup, its agents are available for nearly anything including OpenWrt, it can serve the simplest use case of “is it down” as well as much more complicated ones that stem from its ability to collect data over time.
Personally I’m monitoring:
Is it up?
Is the storage array healthy?
Are the services I care about running?
I used to run it ephemerallly - wiping data on restart. Recently started persisting its data so I can see data over the longer run.