Writing a summary of some aspect of your thesis or research before the viva can do a lot of things to help you. It forces you to focus on something, to highlight the best parts or the most difficult sections and can really support you as you fine-tune your thinking.
It’s important to also recognise that creating a summary can help your memory too. It helps embed ideas. You don’t need to memorise your thesis, or a list or a page of notes, or anything like that. Your examiners want to talk to a person and hear their research, their story and what that means. They don’t need you to recite your work to them.
The focus of writing a summary can help boost what you remember for the viva. You know enough and have done enough or you wouldn’t be working towards finishing your PhD. A little more work can help you remember what you need for meeting your examiners.