The mysterious curse on the Lady of Shalott is a big part of the plot. It rules her life and causes her death. This little thread of black magic helps give "The Lady of Shalott" its spooky, sad atmosphere, and also connects it to the medieval fantasy world of wizards and spells. We can just tell that, if Tennyson were alive now, he'd be a huge Harry Potter fan.
By eliminating specific information about the curse, Tennyson focuses our attention completely on the lady and her loneliness. This makes this primarily a human story with magic elements, rather than the other way around.
By associating the weaving of the Lady with magical power, the poem suggests that all art is a form of magic, a way of casting a spell.