This article talks about the benefits of both manual pumping and using an electric pump.
Snippet:
"In the study, 67 new mothers of premature infants learned how to combine an electric breast pump with hand-expression techniques to extract milk. Unlike prior research showing poor milk production in preemies’ moms, the subjects who used both hands and pump established plentiful milk supplies. By the end of the eight-week study, their average milk production exceeded the amount needed to feed a healthy 3-month-old, even though none of the women studied could nurse when their babies were born. The findings could have implications for women who have full-term infants, too."