As a professional your clients expect to be able to rely on the advice you give. A client who is dissatisfied with your advice or service may well pursue you to recover their loss.
Professional Indemnity Insurance provides you with protection in this event paying both costs and any damages you may be liable to pay.
For example an overseas client is relocating to England, they decide to buy rather than rent, once a suitable property is found their offer is accepted and they then proceed incurring the usual costs of purchase. On the day of exchange they are gazumped and lose the property.
You had not advised them that this was a possibility under English property law and therefore they claim against you for their costs.
Your professional indemnity insurance will deal with this matter.
You also run the risk of being claimed against for damage to property or injury caused or alleged to be by you.
For example whilst carrying out a viewing your coat catches a vase which falls to the ground shattering; the property owner holds you responsible.
Your public liability insurance will deal with this.
The world is becoming ever more litigious and the legal costs of a claim let alone any damages could cripple a business.