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The diagonal conjugate measurement in pelvimetry is primarily used to assess the size and shape of the pelvic inlet, which is critical for determining the potential for a successful vaginal delivery. Specifically, it measures the distance from the lower margin of the pubic symphysis to the sacral promontory, which provides a good estimation of the pelvic inlet's dimensions.

This measurement is significant because it helps to evaluate the space available for the fetal head to enter the pelvic cavity during labor. An adequate diagonal conjugate implies that there is sufficient room for engagement, aligning with the anterior-posterior dimension of the pelvis.

The diagonal conjugate can also help approximate the true obstetric conjugate, which is the true measurement of pelvic capacity and typically more relevant in labor assessments. However, the most direct and immediate use of the diagonal conjugate is indeed to measure the anterior-posterior diameter of the pelvis, underscoring its importance in obstetric evaluations.