If you are a Plumber, Electrician, Heating Engineer or any of the other types of businesses that do work at a customers house then you must have Public Liability Insurance.
Public Liability Insurance covers you in the event that a customer sues you for what ever reason. For instance as a plumber you may accidentally allow a water tank to overflow and cause damage to floors, ceilings, carpets and furniture. In this situation without Public Liability Insurance you would find yourself in a lot of legal and financial trouble.
Public liability insurance covers legal fees and other expenses to do with defending any claim from the public. The insurance company will pay the claim and legal fees.
The law says that if you cause damage to someone or their property then you will have to pay the claim amount which could run to millions of pounds.
Perhaps you are an electrician and somehow manage to short circuit the customer’s computer, this is a situation that the customer could claim against you.
Also the customer may claim against you, rightly or wrongly long after you have completed the job.
With Public Liability Insurance you are protecting your business against going bankrupt in the event of a claim.