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One argument for deontological (or superficially deontological, such as rule-based utilitarian) ethics is the following.
It is useful for one human A to be able to predict that another human B will behave according to certain constraints. It is useful if this prediction can become stronger and more precise if A believes that B intends to act ethically, and especially if B has told A about what system of ethics they intend to follow.
However, utilitarian ethics make this difficult. To predict what constraints B will operate under, A has to emulate the utilitarian computation that B will perform, including an assessment of possible action alternatives, possible outcomes, probabilities, how those probability vary conditionally upon each option, and what the relative values of potential outcomes are.
Deontological ethics make this simpler, especially if 'bright line's are valued in B's ethical system. In this case, A merely has to know what the rules that B follows are, what the possible action alternatives are, which action alternatives break which rule, and, if every alternative breaks a rule, what the rule balancing procedure is. If there is at least one alternative which does not break any rule, then A can be fairly certain what B will perceive their ethical constraints as being.
For example, consider the case where B is testifying in court about the behavior of C. B believes that C is a bad person and that it would be good if C were to be sent to jail. Will B lie in such a way as to make this outcome more likely?
If B is a utilitarian, this is a complex calculation -- HOW bad is B? How much will lying influence the outcome of the trial? What is the chance of being caught? What negative value should be attached to being caught -- what is the value magnitude of B getting punished for lying? What is the value magnitude of setting a bad example for others?
If B is deontological, it might be simpler. Do B's ethical rules include a prohibition against the action of lying in this context (which would most likely be realized as a prohibition against lying)? Do B's ethical rules include a prohibition against the action of telling the truth in this context?
