This piece of farm machinery hasn't been used in my life. I used to think it was a soil aerator similar to what left little dirt dog turds all over the school grounds. But, as you can see in the second picture, the teeth aren't hollow, so it wouldn't work like that.
This is actually called a rotary hoe. I've heard some people use the same term for a rototiller, but they're different machines serving different purposes. Rototillers dig much deeper and are used before planting.
When you plant crops the seeds usually end up two or three inches in the ground. The weeds, however, start growing right at the surface. The teeth of the rotary hoe just barely get into the soil. As you blast through the field with this behind your tractor it just destroys the first half inch or inch of soil and tears up the weeds while leaving your crops wondering what the fuck just happened, but otherwise fine.
Another use is for dry spells. After a long bout of no rain, the wind will kick up and start blowing away your soil. There's several ways to deal with that1. This solution is to pull the rotary hoe across the wind (i.e. if the wind comes from the north you drive east/west) and turn over the soil to bring some moisture to the surface. Then skip 2-3 rows and do it again until the whole field is covered with stripes. Not only will the moist stripes not blow away, but they'll catch much of the dust before it takes to the air.
When I was a kid, we used a springtooth for that. Maybe we'll get to that piece of machinery later. These days the farm is mostly no-till so the fields generally don't get bare enough to blow away.
You'll notice the four flat spots above the whirling teeth. You can throw weight on them if you want the teeth to sink further into the ground.
1 Another solution for preventing soil from blowing include planting wind rows. That's rows of trees similar to the one in the background of the first picture.
These days we see a lot of people planting turnips so their leaves will protect the ground as the soybeans get ready to take off.