It's the law! Plagiarism is theft of someone else's ideas and the words they use to describe those ideas. The words used may contain facts that are obscure, statistical evidence, charts, graphs, drawings and even unfounded opinions. If you use someone's else's words or images you need to give that person credit, or the result is theft. The expression "to coin a phrase" indicates that words are property and have value. Hence, they are protected by copyright just like music, art, or any other creative act. That © belongs to you and to me!
It's a matter of trust: rearranging someone else's words in a paraphrase or summarizing their ideas by precis is not a creative act. If you cannot copyright what you write, give credit! That is how trust in the academic process is built. Would you trust someone who continually ripped you off? Build academic trust and confidence in your own work by citing all material that is not "yours".
It's a matter of honor: Our social fabric and civilization are build on the concept of honor. We honor the past by citing earlier contributions to culture. We honor the present by respecting the work of others who contribute to the academic process. We honor the future by good practices in the present. Please cite all your research! Honor is personal. It has its basis in truth. Being truthful is essential to the academic freedom that guarantees our democratic governance.
See the LibGuide Copyright: What You Need to Know by Lisa Maine and Brian Baker.
Honesty is the human quality of communicating and acting truthfully. It is related to truth as a value. This includes listening, and any action in the human repertoire — as well as speaking.
Superficially, honesty means simply, stating facts and views as best one truly believes them to be. It includes both honesty to others, and to oneself and about ones own motives and inner reality