Stop that bracken
When bracken grows in the kind of soil conditions it loves, it becomes a highly invasive species. To stop it thriving at the expense of other flora, the rangers cut it down during its strongest growth phase. This weakens its underground energy store (which is also known as its rhizome).
Don't forget to rake
If the cut-back bracken wasn't raked away, it would decompose in-situ, adding nutrients to the soil. The habitat conditions the rangers are attempting to create are low-nutrient. This is beneficial to a wider diversity of flowering plant species; if the soil was high in nutrients as a few dominate plants would flourish instead. A good deal of effort is required to strip the nutrients from the cliffs by raking the bracken cuttings away and removing them from site - a good day's work by the team.