Confusion over Medication after treatment and full hysterectomy

Hello.  I'm new to this forum, so i'm not sure where to post this question or issue.  I was diagnosed with IDC in the summer 2011.  I completed all of my surgeries, chemo, and herceptin treatment by August 2012.  I went without a period for 13 months and then it returned.  It really worried me returning because I thought my body was done with estrogen.  My oncologist was very dismissive and said that I do not need to consider a hysterectomy because the medication she prescribed is all that I need.  She had me taking tamoxifen.  In August 2013, I had my annual checkup with a new-to-me OB/GYN.  They found 6 fibroid tumors in my uterus and cysts on both ovaries.  She recommended a full complete hysterectomy with my IDC history.  She said to me that after a hysterectomy, I shouldn't have to take tamoxifen because that is horomonal therapy and my estrogen would be gone. She did recommend that I verify that with my oncologist.  I chose to have the full complete hysterectomy including my ovaries, and the surgery was completed last week 9/11/13.  I called my oncologist and they said to continue on with the tamoxifen until my next appointment in November.  At that point they will change me to post-menopausal medication.

My question today is what is the purpose of continuing the medications?  Is it directed at more than just estrogen?

Thanks for your help.