Why Anastrazole if I Had a Bilateral Mastectomy?

I was diagnosed with DCIS and IDC, Estrogen + in my right breast in November 2024. I had a bilateral/double mastectomy with sentinal lymph node testing in December 2024. Cancer tumor was less than 8mm in size, and all tissue removal edges and lymph nodes were negative for cancer. Since I had both breasts removed, why do I need to take Anastrazole for 5 years? I'm already dealing with hot flashes from menopause and the Anastrazole makes them far more frequent.