Dynamic stretches are part of an effective warm up because they can

Unlike Static Stretching that is recommended for AFTER an exercise or athletic activity, Dynamic stretching can make a good warm-up before you are ready to preform. In dynamic stretching, you increase blood flow, increase the range of motion, and your joints get ready for movement. It focuses on different muscles.

Basically, doing dynamic stretching(which is moving your body while stretching), your body is prepared to move in actual athletic activity.

They can help with the main exercise by reducing the pain after the main exercise