The word aṣlaḥ denotes what is more beneficial or fitting for someone. The contested Muʿtazilī principle turns that comparison into a claim of rational obligation: God must do what is best for His servants.
Example: imagine two possible courses for one person. On one course the person obeys and receives reward; on another the person sins and is punished. Must God necessarily arrange the first course?
The Ashʿarī objection is not that goodness or wisdom lack meaning. It challenges the inference from “this seems more beneficial” to “therefore this action is obligatory upon God.” Comparative benefit alone does not establish such necessity.