Appendices are, as the word implies, extras. This more detailed information is not necessary for understanding the work described in the report, but it can be useful to the reader. For example, a reader may want to know which questions were included in a questoinnaire, perhaps to check whether he agrees with your conclusion based on that, or because he wants to reuse this questionnaire himself. Appendices are often used to keep the report itself concise for the reader, but still offer this extra information for those who are interested.
Common appendices include:
Questoinnaires used, with a brief description included in the report itself to give an idea of the content, and a reference to the appendix should the reader wish to see the complete list of questions.
More results, analysies, elaborations, for example a table of all the results before a particular analysis has been applied, a more complete table with also the less relevant results after analysis (if a shorter version is included in the text with, for example, only the top 10), or transcripts or notes from interviews. It may also concern, for example, detailed designs. The report itself then contains part of these results with a reference to the appendix. In the report itself, some sort of summary can be included, a few examples, or just those results that were most important in coming to a conclusion or advice.