Recalling standard results in a master's thesis appendix for reader's convenience and/or self-containedness
My answer is very similar to the one I gave to the question I linked to in a comment:  There is usually no reason not to include something in a thesis.  Your thesis is a place for you to give a coherent account related to your work on a topic.  There are (usually) no page restrictions, and it should serve as a reference document for future readers.

Many theses include a lot of review material, beyond just what might be contained in a literature survey. A thesis has to contain new material, of course, but it does not all need to be novel, and there can be a lot of merit in giving detailed explanations of known results, if the readers are not necessarily going to be familiar with them in detail.