Insurance for your business assets is absolutely essential to ensure you are able to survive an insured catastrophe. Insurance protection of your Turnover or Revenue, is equally essentially to ensure business survival, as it is designed to cover your ongoing expenses (like salaries, rent etc.), as well as making good your expected Net Profit your would have earned, but for the insured catastrophe.
This cover essentially only triggers, if an event covered by the Fire section of the policy occurs, which causes an insured reduction in Turnover/Revenue.