What is the difference between screening mammogram and diagnostic mammogram? Why does the diagnostic one have to be done with breast ultrasound?

Purpose. The purpose of a screening mammogram is to identify breast cancer in women who have no signs of disease, while the purpose of a diagnostic mammogram is to obtain more detail on a known abnormality identified on prior physical exam or imaging study. A diagnostic mammogram need not always be done with an ultrasound.
A screening. mammogram is a study of both breasts that is performed on asymptomatic women to try and detect signs of breast cancer. A diagnostic mammogram consists of special views, and is usually targeted to a specific area of the breast, to further evaluate a potential abnormality seen on screening mammo or felt on clinical exam. The diagnostic doesn't have to have an US, but US usually very helpful.