First of all, it reinforces the importance of their desire for "precision of language." Most societies expect that toddlers will make mistakes as they learn their language; small children confuse words all the time, and we generally take it in stride and provide simple corrections. In this community, however, when Asher makes a childlike mistake in language, he gets what he asked for - a smack. It makes me wonder what would happen if he were to accidentally ask for a "stab."
The harshness of the incident seems apparent to a modern reader. However, in that society, precision of language is important for maintaining the peacefulness of their existence. For example, later in the novel we learn that Jonas is corrected for using the word "starving" when he really means "hungry." The community members want to keep certain uncomfortable or unpleasant words out of the collective vocabulary; in order to do this, it is necessary to ensure precision of language usage for every individual member of the community. The incident with Asher illustrates how they maintain this; the incident with Jonas, later, shows us why.