The sun's face is many times larger than the Earth's. The light from the sun reaches us as parallel beams. When it's a bit cloudy with gaps between the clouds the beams seem to come from a light source (the sun) just up in the atmosphere and the beams are spread out at angles as if the light source is just a short distance up. Surely they should just come down in parallel with each other?
You are absolutely right about the Sun's rays coming towards Earth as parallel rays! So then why do we see the rays appear to converge at a point source sometimes? The answer is - it is just a matter of perspective. For example, consider standing on a long straight stretch of railroad tracks. The tracks are indeed parallel to each other, but appear to converge at a point the farther away you look. It is the exact thing going on with the rays of the Sun! Hope this answers your question!