A service agreement is a document that makes sure you and your provider are on the same page. It will say what they will offer you, how they’ll work for you, and also what expectations they have from you (e.g. giving notice if you want to leave).
Although they aren't mandatory (unless for SDA support), they make clear what you have both agreed to in writing. Service Agreements are also recognised by Australian Consumer Law.
They include information such as:
Here's a link to a page on the NDIS website that talks about some of the things to consider when making a Service Agreement.