Usually for staging.
From diagnosis: Biopsy confirms breast cancer, breast MRI to check lymph nodes. If lymph nodes look questionable, lymph node biopsy is done. If positive, staging is done. Depending on insurance either a PET scan or a CT of the chest/pelvis/abdomen to determine if the cancer has spread.
For stage IV: staging every 3 or 6 months to monitor for response to treatment versus progression.

I didn’t get one. I had a CT scan and a bone scan.

I got my PET scan once a CT scan noted it spread to the sternum to find out if it spread elsewhere. 3 different areas showed positive.