Answer: It gives the speaker the quality of empathy to show that he is open to the pain of other Vietnam veterans.
Step-by-step explanation:
When the speaker says "he is a window", (he is not literally a window cause he's using) a metaphor. What he means is, just like light enters a window, those images enter his eyes, mind, and heart. He is empathetic to the suffering to those people, and he has changed now forever affected by what he has seen