This allows the soil to absorb moisture and oxygen. It can be done in Spring or Autumn and when this is undertaken will depend on the presenting issue:
September and October being the best time to remove heavy thatch and moss, and over-seeding.
Spring however is a good time to do a light scarify (mechanical spring rake) as part of a regular thatch management programme.
Over-seeding can also be done in Spring if not too close to the Summer heat.