The unmanaged lawn in the picture has a moss problem!
Raking the lawn in the spring with a wire tine rake will help remove moss by pulling it out and break through the lawn thatch. The gentle action of the rake compared with the scarifier is less likely to cause damage to the lawn just as the weeds are becoming numerous.
Scarifiers use metal blades to cut through the thatch and are much more aggressive and as such are suited to use in the autumn. Scarifying results in a firmer lawn that allows the passage of air, nutrients and water easily into it. The scarifier blades cut into the soil and open the soil surface and also ‘prune’ the grass plants because it cuts them downwards rather than cross cutting as in mowing. This pruning, just like pruning a bush or plant causes extra shoots to grow, thickening the turf.