The principle of fairness and the role of personal recognition within the reward system of science account for the emphasis given to the proper allocation of credit. Conflicts over proper attribution can arise in any of these places.
Citations serve many purposes in a scientific paper.
They acknowledge the work of other scientists, direct the reader toward additional sources of information, acknowledge conflicts with other results, and provide support for the views expressed in the paper. More broadly, citations place a paper within its scientific context, relating it to the present state of scientific knowledge. Failure to cite the work of others can give rise to more than just hard feelings. Citations are part of the reward system of science. They are connected to funding decisions and to the future careers of researchers.