Ceiling insulation can settle down over time and this reduces its efficacy in keeping warmth in during winter. If it has compressed to less than 50mm, topping up helps.
Gaps in insulation also reduce the efficiency of existing insulation. A 10% gap can reduce the overall R-value of insulation by 50%. For example, the area above downlights, left uncovered usually due to fire safety concerns, results in insulation gaps. Downlight covers can be retrofitted to allow better coverage of insulation around these lights.