I read on the package insert that the use of progesterone isn't necessary if you've had a hysterectomy. I am a strong proponent of hormone replacement therapy, but in your case, I think only Premarin, the prem in Prempro, or an equivalent estrogen, would be necessary. Your doctor may think that the progesterone will increase your libido, which often falls after a hysterectomy or after menopause. But I don't believe there is any evidence that this helps. Some doctors do give a testosterone supplement to women for this reason.
Many women swear it has reinvigorated their sex lives.
Your doctor may also think that progesterone decreases your likelihood of your developing breast cancer. But again, I don't think the evidence is convincing, and other forms of estrogen that will reduce this risk will soon become available. A recent, large study appears to confirm the fear that taking estrogen after menopause does increase breast cancer risk slightly the longer one takes the estrogen. But the study also demonstrates that taking estrogen reduces the risk of dying of heart disease. The decreased risk of dying from heart disease was much greater than the increased risk of dying from breast cancer, even in women at high risk for developing breast cancer.