et vero quisquam Iunonem laedere nolit
offensamque tremat, quae prosum sola nocendo?
quae prosum sola nocendo This is a little obscure but I think the sense is that Juno alone benefits others when she tries harm them. As huilen notes, prosum here means "to benefit others," not "to be benefited." Juno has switched from talking about herself in the third person to the first person.
Juno has changed Callisto into a bear--Callisto's crime was to have been raped by Zeus and to have given birth to a son, Arcas. At age 15, Arcas is out hunting one day and encounters his mother. Callisto recognizes him, and hesitates to attack her son, but Arcas doesn't recognize her and is about to kill her, when Jupiter transforms them both into constellations. When Juno sees this, she is indignant. Her efforts to punish Callisto have resulted in Callisto's catasterism. She has benefited Callisto by harming her.
Ovid's characterization of Juno is consistent with, and reminiscent of, Vergil's characterization of her in the Aeneid, as jealous, indignant and arbitrarily punitive. But in Vergil, her jealous rage is godlike; in Ovid, she is a comic figure, the object of Ovid's irreverent humor. There is a large dose of Vergilian parody in this.
The story of Callisto is told in the first third of the Metamorphoses, in which a major theme is the "crimes of the gods." The male gods are lecherous and aggressive, raping nymphs and mortals right and left (other examples are Jupiter and Europa, Apollo and Daphne), while the goddesses are jealous, cruel and arbitrarily vindictive (Diana and Actaeon, Athena and Arachne). As Juno is here, the gods are in large measure comic figures. As Ovid moves into the second third, the stories are more concerned with human suffering, largely through love, and the human figures are portrayed with deep sympathy, even when they are tormented by illicit passion.