Just is "just" a command runner, it is designed to handle the cases where Make is used to formalise commands and dependencies. It can also handle build commands but that isn't its focus.
I think the biggest argument for moving to Just is if your .PHONY target includes most of the targets in your Makefile.
It also allows positional arguments to passed to the tasks, which something I would have love to have done with Makefiles in the past. I'm so used to the workarounds now that I can't remember which files I should revise now it is possible.