Because every pixel is normally only sampled once they cannot get the correct output if the whole pixel isn’t covered by one triangle. This is beacuse the sample can only get it’s colour (and other properties) from one triangle. In the contour of a model this is very apparent and if no multisampling is enabled then the edges will be jagged and appear very annoing. This effect is called aliasing and to deal with it we would need unlimited with samples in the pixel and blend them together, but it’s not possible with the current hardware. Instead we have to accept only taking two, four or more samples and hope it’s enough. There are a lot of different methods to do multisampling and the table below show some of the most used ones and the result from using them on a filled triangle and a outlined triangle.

Image from the book Real-Time Rendering, used with permission.