By including specifications in an appendix, you can add credence and understanding to something that you've referenced in your report. For instance, if you've written a report about gas-efficient vehicles, your audience may find it interesting to read the specifications for gas-efficient and non-gas-efficient vehicles. This can help illustrate differences and similarities between the two and may help the reader understand more difficult concepts within your report.
If your report is based on survey data, as many are, the inclusion of the actual questionnaire used to garner the data would be very helpful to those reading your material. This way, when you reference a question in the report, the reader has a source to refer to, allowing him to see how the question was phrased and what follow-up questions were posed. The ability to see an entire questionnaire and not just selected portions can raise comprehension of a topic.
If you charted data and saw some patterns, don't just talk about it. Show what you've found. Include tables and charts in an appendix, and your readers may be able to see the correlations for themselves and respond genuinely to your report. Some people understand concepts more clearly when the information is laid out in front of them, and others understand better by reading about it. Either way, the inclusion of tables may add credence to your report's message.
By including complete transcripts of any relevant interviews that took place while conducting research, you lay yourself and your research methods out for all to see. When presenting reports, that can be a good thing, especially when the interviewee gave relevant information that was too lengthy to include in the report itself. By including transcripts of any important interviews conducted, the reader can flip to them if she wants more information from a particular source.