For Invoices , what does the "Renew" button actually do? I mean, if I click it, it takes me to Estimates. I am just trying to work out what it is actually renewing? Looking at SA, it seems to be something to do with renewals of contracts. But we don't have any of that stuff enabled.

Hey, this button is tied to renewal items and the contract section. If you had things enabled I remember the button automatically creating a transaction you can save (estimate, sales order, invoice) to renew the services on that specific invoice.