Professor said: Rasterizer is a ray tracer that is very efficient for particular set of rays. What does that mean?

Rasterizer can put all objects onto a screen in front of a pinhole camera, and determines, for each pixel, which object is closest to it. Thinking reversely, for each pixel on the screen, the rasterizer is looking for an object, on the ray of camera->pixel, that is closest to the camera. So rasterizer can be treated as ray tracer, only for the rays from one point (camera) and through a grid of pixels.