An acknowledgment in a dissertation is usually included to thank people who have assisted you throughout the research and writing process. For example, an acknowledgements section might include your supervisor, mentor, or even family or friends who provided moral or financial support. You may also include institutions who funded your research, librarians, lab assistants, and editors. Finally, you might want to thank those who provided inspiration or guidance along the way.
Including an acknowledgements section in your dissertation is a way of saying thank you to the important people who helped you in the process. After all, a dissertation is a huge undertaking and involves a great amount of effort and hard work. Taking the time to thank those who contributed to your success is both polite and professional.