According to the narrator, what is the veil's one desirable effect?
Though the veil has a number of tremendously negative effects, it does have one positive or desirable consequence as well: it makes "its wearer a very efficient clergyman." Wearing the veil, Mr. Hooper "became a man of awful power over souls that were in agony for sin." It seems to make converts, those who have led particularly sinful lives prior to their conversion to Puritanism, feel a great deal more comfortable to know that their minister "had been with [them] behind the black veil." Typically, such a person might feel very awkward or embarrassed to divulge their darkest sins to a minister who they feel cannot possibly relate to or understand the choices they've made in their past; however, when faced with a minister who they feel can understand them, because they have proof that he has committed his own sins, they would likely feel a great deal more relaxed.
Further, "Dying sinners cried aloud for Mr. Hooper, and would not yield their breath till he appeared." Again, sinners feel a sort of kinship with Mr. Hooper; perhaps they feel a fellowship in the knowledge of their shared sin, especially when everyone else tries so hard to hide their own sinfulness. These open sinners do not have to feel so isolated or alone when they have their minister nearby.