Why is it necessary to get so many ultrasounds in pregnancy/

It isn't. In an uncomplicated pregnancy, repeated ultrasounds aren't necessary. A 1st trimester sonogram is best for confirming the due date and number of fetuses while fetal anatomy is best assessed in the mid to late 2nd trimester. Additional sonograms may be recommended if complications such as hypertension, diabetes, or pre-eclampsia arise, but for the uncomplicated, low risk patient, 2 should suffice.
Development. Make sure fetus is developing and growing normal. Placenta has not separate from uterus.