This may be an extreme example, but moss in a lawn can quickly take over and spoil it if it’s left untreated, and there are a number of common causes for the arrival of moss in a lawn …..

Soil Compaction

Soil compaction in a lawn frequently occurs when people walk in the same places over and over again – like people do when they mow their lawns, always starting and finishing in the same places for instance. This tendency is made more likely if you have a heavy soil with a high Clay content.

Combined with cutting too low and maybe having areas of a lawn in heavy shade for most of the day, these two factors alone can ‘open the door’ to Moss, because compaction equals loss of air in the soil, and loss of air in the soil equals poorer drainage. Cutting too short allows moss spores to get a hold through less competition. If you must cut the same way every time, then make sure to spike and aerate your lawn in the autumn. This will help re-oxygenate the soil, stimulating a healthier root system as a result, as well as improving drainage.

The Fifth point I would make refers to maybe a complete lack of any lawn treatments.

These could include Spiking or Aerating and applying fine grit, Scarifying, Feeding, etc., etc.. However if you ignore the previous four observations, you will be expecting a miracle from your chosen treatment, because it’s always better to start as you mean to go on.