To give credit where credit is due. So that readers can go back to the original sources of facts and opinions for themselves.
You cite so your readers can trace your intellectual footsteps back to the source of the information. Citing also serves as a professional courtesy to the researchers whose discoveries and ideas you are using. But citing has nothing to do with copyright. You can cite something and still be violating copyright, or you can legally copy something in the public domain and still be committing plagiarism.