I have an appointment with a reproductive endocrinologist so why do I also need an ob/gyn?

Doctors. I am assuming that your visit to a reproductive endocrinologist is for infertility. After you conceive, you will need to see an obstetrician (ob/gyn).
Routine ob gyn care. Reproductive endocrinologist are specialist. They can help you get pregnant but you will need OB gyn for your pregnany care and delivery. Also your annual exam, pap test etc after the baby is born.
Good question. If you are discussing infertility issues with a reproductive endocrinologist, their goal for you is to achieve pregnancy. Once pregnancy is safely established, your care is frequently turned over to an obgyn, who manages your pregnancy and delivers your baby. Most reproductive endocrinologists do not manage pregnancies although they will often keep in touch and make sure you are doing well.