In the old days, artists would change the scenes to match the technology. If human skin couldn't be rendered well, you just showed off plastic and metals. Or, they'd make a "cartoony style" and hope that the audience would get used to the plastic barbie-looking skin.
Or in the case of Toy Story: you just make a story revolve around plastic toys. Plastic looks good, remember, in 90s CGI. As long as actually humans are rare, the movie will look "realistic".
But you can't just keep making movies about Toys with plastic-like skin. And improving the algorithms to have realistic skin requires harder calculations such as "subsurface scattering".
Today, artists want to show off more materials, and they also want all those materials to be more believable. Pixar is insane with their details: they spent two years on creating the "Hank the Octopus" in "Finding Dory". And it shows: good reflections off the skin highlight the wetness of the Octopus, Hank walks and moves very much like a real octopus, etc. etc.
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In short: there are some materials which don't require much processor time. You probably can get away with a toy-story like plastic look and feel with 10 samples per pixel (talking 90s era here, where things were still relatively flat and primitive).
But if you want to create a CGI Grand Moff Tarkin in an otherwise live-action movie (Star Wars: Rogue One)... you'll need to do better than that. These scenes will naturally require 100s or 10,000 even samples before the human eye is tricked.
I get that doing 100 Ray per pixel gives you quality that an expert need to look hard to find defects given expensive calibrated displays in well lighted controlled environment. But a normal user isn’t doing that. Their room lighting is already washing out on several artifacts. Their TV sets are not even properly calibrated or even 4K. For those common people, what is good enough?