As a builder you will understand how crucial it is to have a public liability insurance policy to cover you while you are working. You will want to insure your liability for all eventualities. Many contractors will not let you work or release your payment until you can provide proof of your liability insurance.
An example incident where your public liability insurance would usually cover you is if you were to fit a bathroom and while fitting it you caused a flood in the property. Obviously without this peril being insured you could be inline for paying a very large bill.