Scarifiers basically scrape away moss and death thatch which is the remains of last year’s growth. Doing this, scarifying, is just one thing you need to do if you have lots of moss.
Moss forms when the soil is continually heavily damp and compacted. What you need to do in this instance is use an aerator – basically a set of hollow tubes that lift plugs of soil out of the lawn, brush in sand in to the holes so formed, spread combined moss killer and lawn feed then a couple of weeks later scarify to remove the dead moss.
You can hire aerators – unless you have a huge lawn it’s not worth owning one – but if you’ve a small lawn then you can get the same effect by using a garden fork: stand on fork; wiggle back and forth a couple of times to make a hole then move on a few inches and repeat, another good workout.