You're obliged under Electricity at Work Regulations (I believe, someone will correct me if I'm wrong I'm sure) to ensure the safety of your staff and members of the public who enter your premises.
A PAT testing regime can help demonstrate you're fulfilling your duties by ensuring the portable equipment is safe for continued use. Without PAT testing records, if the unthinkable happened and someone was injured or killed as a result of an electrical fault, it could be argued that you've been negligent.
The same goes for the fixed wiring and equipment of any premises you're responsible for where business is conducted. This is why it is recommended that you have an electrical installation condition report produced periodically (the recommended periods vary depending on the type and use of the premises).