The author inserts this information during the scene at Miss Strangeworth's home when she is writing three of her anonymous letters. This is to inform the reader that these and similar letters have no basis in fact and to illustrate that this old lady is only imagining "the possibility of evil."
Miss Strangeworth never concerned herself with facts; her letters all dealt with the more negotiable stuff of suspicion.
She writes one letter to Don Crane suggesting that his six-month-old baby girl might be retarded. She writes another to Mrs. Harper suggesting that everybody in town knows her husband is having an affair with a woman she does not identify by name. And she writes a third to an "old Mrs. Foster" suggesting that her nephew might be planning to have her own doctor kill her on the operating table. Therefore the reader knows that all Miss Strangeworth's letters are baseless. They are not deliberately false. Miss Strangeworth imagines the possibilities she suggests in her letters. She is obviously mentally deranged. She doesn't realize how much trouble she is causing. In fact, she probably thinks she is being helpful. For example, in the letter to Don Crane she asks:
Some people just shouldn't have children, should they?
This is her way of suggesting that the Cranes, who have only one child, should not have any more.
In her letter to Mrs. Harper she is only suggesting that maybe the woman should keep an eye on her husband. And in her letter to old Mrs. Foster she is suggesting that she ought to think twice about her planned operation. The reader can imagine how all three of these letters would worry the recipients. No doubt Mrs. Foster would already be worried about having a major operation if she is old. The recipients would probably all think the anonymous letters were based on some factual information.
Many people seemed disturbed recently, Miss Strangeworth thought.
She had been writing these letters for the past year, but she doesn't realize that people all over town seem disturbed because of her. She sees the trouble she has caused young Linda Stewart and Dave Harris, but she assumes this is because she hit the nail right on the head when she wrote to Linda's parents suggesting that the teenagers might be having sex. She probably assumes that if she writes a lot of letters, a certain percentage of her suggestions will prove correct.
Another fictional character liked to send poison-pen notes. He was Jim Kendall in Ring Lardner's story "Haircut." The barber tells his customer:
Jim had a great trick that he used to play w'ile he was travelin'. For instance, he'd be ridin' on a train and they'd come to some little town like, well, like, well, like, we'll say, like Benton. Jim would look out the train window and read the signs of the stores.
For instance, they'd be a sign, "Henry Smith, Dry Goods." Well, Jim would write down the name and the name of the town and when he got to wherever he was goin' he'd mail back a postal card to Henry Smith at Benton and not sign no name to it, but he'd write on the card, well somethin' like "Ask your wife about that book agent that spent the afternoon last week," or "Ask your Missus who kept her from gettin' lonesome the last time you was in Carterville." And he'd sign the card, "A Friend."