Grass harrows come in two types chain and tine. 

The purpose of harrowing is to rip out the thatch of dead grass and moss, this allows any new growth to access the available air and light. 

Harrowing is also used to break up and disperse dung piles, especially when horses have been grazing. 

Chain harrows have traditionally been used, but modern tine harrows are less prone to "clogging up" so tend  to do a better job.