DE can be used to prevent external parasites on your chickens, their roosts, nest boxes, etc. Some people use it in gardening as well to deter slimey, crawling things. It certainly deters flies from what I've noticed.
Others use it as supplemental feed for their animals in hopes of deworming them 'naturally'. It's not harmful to the animal; but there is no scientific studies supporting thta it does anything for internal parasistes. External paraistes, though, it has been shown to control.
If it were free or cheap, I'd say "use it, it's cheap insurance". But, it's rather spendy stuff. I'd recommend saving the money, doing fecals on your livestock, then getting the appropriate antihelmentic for your livestock.
