I was confused as well, and then received the missing piece of information from the threejs website.
Right, so its like if they are just parameters, why aren't they just like a material. PBR is a "physically based material" is a specification around specific parameters that can be used to achieve a certain kind of realistic look that is compatible with a specific kind of technology used in modern renderers.
Really realism could be defined by thousands of parameters, though they are saying use these parameters, and these parameters only to create a material, and then our rendering technology will be able to reproduce that exact look. A new programming paradigm that has emerged also has to do with the way rendering technology works on different platforms like windows, linux, mobile devices, etc... and the new tech like directx12, vulkan, and metal on apple are similar in how they do rendering on GPU's. So PBR's are now also useful that when viewed on different devices, they will be more similar to original design across all devices, than in the past.
The threejs website gave me the clue that PBR's, once defined, don't require multiple configurations for different lighting conditions like in the past. If you change from a bright light source, to a dimly light room, the material calculations will update automatically because the way ray tracing calculations works. Also threejs says physically correct model as well, which also has to do with light and distance calculations. The light value of a PBR will behave just like a real life version of it will, as far as the distance it emits light or affects other objects in the scene, which is pretty interesting!
With rasterization, the color of the material was more based on the value of the material, with ray tracing is more about the surrounding environment determining what that material actually looks like. PBR is designed give more realistic looks to materials and textures based on ray tracing calculations. Specifications in general, for non-programmers, like jpg, png, PBR, etc... are used so that multiple companies that make different tools develop a standard way of storing and using data in efficent, replicable manner. Also that way, something can be improved upon independently, and then updates can be provided to the rest of the industry on that standard when needed.
So based on the community and industry's excessive amount of work done, new technologies have been developed and PBR is a new way of interacting with those technologies with respect to material and textures on 3D models.
