Answer: They wanted to get her to change her mind about leaving the Opera Populaire for good.
The Phantom's notes hadn't really frightened Carlotta at all. The first note she received merely annoyed her, by thinking that the Vicomte had the gall to insult her by replacing Carlotta with Christine. The second note didn't frighten her either, as she launched into a tirade directed at the managers, saying that they didn't appreciate her talent (because they wanted Christine to sing instead of her), that they didn't love her, they had rebuked her, etc. In order to appease their diva, so there would be a soprano who could sing on stage (and because Miss Daae was still nowhere in sight), the managers were shown giving Carlotta gift after gift. They were trying to convince her to stay at the opera and that her talent truly was appreciated.