OP,
I do not think I am fully understanding your question. At first, I thought your question of, "why are these plates here?" was asking, "what purpose do these plates serve installed in the tee?" but now, looking at the pictures closer, I think what you are asking is "Why are plates from a separator, 100 meters away, now lodged in this tee and what purpose do they serve in the separator?" I am considering this understanding of your question because the plates to not appear to be mechanically fastened in the tee and seem randomly stacked, so it looks like upstream debris that became lodged in the tee. Maybe I am the only one seeing this and if so, I apologize for any misunderstanding.