How could this be a good thing? Isn't it bad for tissues to not get enough blood flow?
Actually, you don't have enough blood for all your tissues. You open and close your arteriolar sphincters all the time, so your tissues can take turns having the blood.
When you eat, you dilate the arterioles in your GI tract and send blood to help with digestion.
When you exercise, you dilate the arterioles in your muscles and send blood to help them make ATP to move with.
And when your blood pressure drops, you constrict the arterioles in your GI tract, your skin, and your kidneys. This raises the blood pressure in your major arteries and sends more blood to your heart, lungs, and brain.