A lawn scarifier, sometimes referred to as a 'dethatcher', is a garden tool that is designed to cut through the soil, helping to remove dead moss and other debris like grass cuttings. The cutting action of the tool, either powered by electricity or a manual push action, also helps to aerate the soil, making it healthier, (almost) weed free and longer lasting. See the video below of the Stihl model.

The consequences of not scarifying your lawn could see your lawn becoming overun with moss. If dead grass cuttings and moss are allowed to sit on the lawn's surface they can prevent rain water from draining deep under the ground where it's needed the most. Much of the moisture will sit on top making your lawn soggy and boggy.
The layer of dead grass that sits on top of a lawn is sometimes known as a 'thatch', which explains the 'dethatcher' reference. Excess moisture can also encourage more moss growth exacerbatring the issue .

• Removes harmful levels of thatch.
• Prunes grass roots encouraging growth.
• Allows water to reach the soil.
• Allows air to reach the sward.
• Allows nutrients to reach the grass plants.
• Removes moss and dead weeds.
• Reduces the likelihood of future moss problems.

Scarifying with the Stihl RL540 Video Link: https://www.youtu.be/TgeVl0x0xYU