ISO9001:2015 is a quality management standard. It provides a number of ideas that help organisations to improve how they make a profit, and to do it repeatedly. It has ideas on everything from sales, purchasing, design, production, improvement, and all other aspects of how an organisation operates, and how it satisfies its customer.
Where organisations demonstrate to a Certification Body (CB), like BSI, that they have implemented these ideas, or clauses of ISO9001:2015, the CB may certify them to the standard.
Most organisations have ISO9001:2015 in order to fulfil a customer requirement, or to be able to tender for a contract. It therefore is a way for organisations to try to improve their sales.
Best Practice organisations go further, using the standard to become better at what they do, becoming more consistent, more effective and more efficient. In doing so their customer satisfaction and reputation grows, they deliver a product or service that more closely meets exactly what the client wants, and they reduce costs. It is therefore a continuous improvement tool.
However, the real benefit in ISO9001:2015 comes from cultural and motivational improvements, where staff are more empowered and happier in their work.