Why does Harper Lee put Atticus under the light bulb in front of the jail and much later in the novel under the streetlight?

I agree with the previous answer.

Atticus also represents the light of truth. He knows the truth and simply sits under the light waiting. He is stragecially situated near the newspaper office.  Atticus is in the open space in plain view.  The truth should be exposed and in plain view.  Atticus under the light is Harper Lee's symbolism of "the truth" of this story.

Harper Lee uses the light to focus the reader on the darkness of the crimes and false accusations against Tom Robinson.  If anyone were to attack Tom, he would have to go through Atticus and through the light.

It was the light that allowed Scout to recognize a familiar face.  Once that occurred, the lynch mob dispersed.  The light also allows Atticus' friend, Braxton Bragg Underwood, to cover him with a shotgun the whole time the mob is there.