The portal systems are insanely important!

Normally capillary beds are just to deliver oxygen to the tissue. The 3 portal systems though have other functions beyond just oxygen delivery.

Hypophyseal portal system delivers oxygen and also acts as the communicator between the hypothalamus and the anterior pituitary. The hypothalamus sends hormones through the blood right to the anterior pituitary. The portal system brings the blood first to the hypothalamus and then right to the anterior pituitary instead of it having to go through the whole body.

The hepatic portal system first hits the gut, then goes to the liver. It not only delivers oxygen, but absorbs lots of things from the small intestine and brings it right to the liver where it can be taken in (take the example of glucose, it gets absorbed then right to the liver where you can make glycogen, instead of it having to travel all the way around the body).

Finally renal portal system. It goes first near bowmans capsule where filtration occurs and lots of the stuff in the blood gest taken out, then the blood travels to the next capillary bed around the nephron where it can reabsorb most of the things that were just taken out of it.