This is the use of a professional petrol driven scarifier to remove surface thatch and dead organic refuse. This layer builds up as part of the normal life cycle of grass.
Unfortunately during the summer this thatch layer works like a thatched roof and keeps out most of the water it receives. Conversely, in the winter it holds the water like a sponge creating the environment moss loves to grow in!

Therefore this is a vital part of your lawn care service.