Urging (التحضيض) presses the addressee to act. Reproach for a missed action (التنديم) recalls what should have been done. Hallā (هلا) and related forms can perform either function; the time of the verb helps determine which.
«هَلَّا أَكْرَمْتَ زَيْدًا»
Why did you not honour Zayd?
With a past verb, the utterance may reproach a failure already committed. With «هَلَّا تَقُومُ»—“Why do you not get up?”—it urges an action to be performed. Reducing every hallā to a neutral question loses the distinctive purpose of prompting or reproach.