As a landlord, your duty is to ensure the safety of the electrical appliances in your property. Having a PAT test carried out is a great way to meet your legal obligations and keep your tenants safe.
From a practical point of view, different appliances need testing in accordance with their risk of breaking. High-use, highly portable appliances like toasters are far more likely to be damaged in everyday use than a stationary washing machine, meaning a landlord might responsibly have the former tested every two years, but the latter every four years.