This HOG1x is on large sump, and is probably too small to do enough filtering, but could be plenty of food-growing if the inside wet part (with growth) is placed into the display once a week for an overnight feeding:
This HOG1x is on the glass of a small freshwater tank, and sometimes the fish will learn to pull and eat the growth out of the circulation holes; if they don't, you can just open the Hog and let the fish in to eat. Snails too, of course.
Here is another small HOG1x on the display glass of a saltwater tank. It's brand new, with black shade cloth still in place. And like other small scrubbers on big tanks, it's probably to grow fresh food for feeding.
This HOG1® or 1x looks fairly new and is getting an early growth of brown slime. After a brushing in tap water, it will probably start growing some green hair algae next.