What is Three.js?

It wraps WebGL in the concepts you would expect: a scene holding objects, a camera looking at them, materials describing how surfaces respond to light, and a renderer producing frames on the GPU. That abstraction is what makes browser 3D approachable, because the underlying API is extremely low level and unforgiving.

The cost lands on the client's device. Models and textures are heavy to download, and continuous rendering keeps the GPU busy, which on a phone means heat and battery drain. So it needs to be loaded lazily rather than blocking the page, and there has to be a version of the experience that works without it — for anyone on a weak device, and for the crawler that will not run your scene.