This is an interesting question in part because most readers conclude that the veil has only disastrous effects--alienating Reverend Hooper's congregation from him; ruining his fiancee's (Elizabeth) hopes for a life as a wife and mother; destroying Hooper's own life as a religious leader--but Hawthorne makes it clear that the veil also has some unexpected, but positive effects, as well:
. . . the black veil had the one desirable effect of making its wearer a very efficient clergyman. . . he became a man of awful power, over souls that were in agony for sin. . . Dying sinners cried aloud for Mr. Hooper, and would not yield their breath till he appeared.
Because some of Reverend Hooper's congregation assumed that the veil represented Hooper's own hidden sins, they felt comforted by the fact that their religious leader was, like them, a sinner who could the nature of sin and sinning.
Later in Hooper's life, he was asked to give an election-day sermon, in part, of course, because people were curious to see Hooper's veil, but the sermon had an unintended effect:
. . . he stood before the chief magistrate, the council, and the representatives, and wrought so deep an impression, that the legislative measures of that year were characterized by all the gloom and piety of our earliest ancestral sway.
In other words, Hooper's sermon, certainly enhanced by the veil, affected the law-makers so powerfully that the laws they enacted that year reflected a return to fundamental Puritan beliefs.
In summary, then, even though the veil certainly had seriously negative effects on Hooper and everyone around him, it provided comfort to some of Hooper's congregation on their deathbeds, and the veil's influence was sufficiently powerful to return the state's law-makers to their Puritan roots.