Sometimes you may wish to include material as a supplement to a document, but would not be appropriate or necessary to include in the main body. This material can be included as an appendix if it would help your reader, such as providing further detail to understand your work or replicate a study you have carried out.
The type of material you might include in an appendix could be:
A full search strategy of a literature search
Full set of data from research you have carried out
Templates of questionnaires/scales/tests as relevant to your document
Other detailed or complex information that you may have provided in a summarised format in the main body