It was likely meant to be a temporary means for the system architect to monitor and improve the system after it was deployed but then life changing circumstances may have distracted his attention away from decommissioning the modem. The movie still holds up today and is worth a watch. Actually it may be more pertinent now than ever.
I think what makes it likable for me is that it's all on the cusp of believability. Obviously LLMs weren't quite mature enough to do everything Joshua did back then (and probably not now), but the fact that the "hacking" was basically just social engineering, and was just achieved by wardialing and a bit of creative thinking makes it somewhat charming, even today.
With the advent of LLMs being used increasingly for everyone, I do wonder how close we're going to get to some kind of "Global Thermonuclear War" simulation gone awry.