How would this help increase blood circulation?
I believe that the answer is in the same cited source (BioSBCC).
Veins are considered a blood reservoir and undergo constriction to mobilize blood.
[...] the veins act somewhat like a blood reservoir, containing 60% of the total blood volume at rest. [...] When the body needs to mobilize more blood for physical activity, the sympathetic nervous system induces vasoconstriction of veins.
The result is an increase in circulating blood volume. This changes cardiac output and arterial blood pressure (CV Physiology: Blood Volume).
I also recommend reading on Biology.SE: Where does extra blood come from to fill your muscles during exercise?
There does not need to be a change in circulating blood volume during exercise; the role of vasoconstriction is more subtle than that.
Vasoconstriction increases the stiffness of venous vessels leading back to the heart. This makes them act more like stiff tubes than elastic reservoirs. As a result, any extra blood pumped by the heart is more likely to return to the heart rather than to be stored in the venous vasculature. The importance of regulating venous return in concert with cardiac function has been appreciated at least since the work of Guyton and colleagues in the 1950s.
In exercise the effective muscle pump arising from the combination of muscle contraction and valves in the veins preventing backflow is also important; both the muscle pump and venoconstriction promote venous return to the heart.