One way of interpreting this quotation is that Daisy is being bitterly ironic. The "home influence" of the Buchanans is hardly a wholesome one. Tom is having an affair, and Daisy and Tom are obviously fighting. There is quite a bit of tension at this house. After this quotation, we are told that Daisy and Tom look at each other "for a moment in silence." This remark obviously adds to the tension of the scene, because Nick hurries to change the subject by asking if Jordan is from New York.
The Buchanan marriage is obviously a sham and would not be a healthy influence on Jordan. Daisy is taking a jab at Tom, and Tom being the guilty one, can only respond in silence.
This line from the novel occurs in a scene after dinner at the Buchanans when Jordan Baker tells everyone good night and retires. After she leaves, Tom observes that Jordan's family should not let her "run around the country" on her own. Daisy points out that Jordan's only family is an elderly aunt. She adds that Nick will be looking out for Jordan. Nick just met Jordan, but Daisy is either matchmaking (she had mentioned earlier pairing Nick and Jordan), or she is simply taking issue with what Tom has said. It is at this point she adds:
She's going to spend lots of week-ends out here this summer. I think the home influence will be very good for her.
More significant than what Daisy says is what happens immediately after. She and Tom "looked at each other for a moment in silence," and Nick quickly asks if Jordan is from New York, his intention being to break the awkward moment between Tom and Daisy.
Daisy is speaking ironically, sarcastically. She knows Tom is having an affair. His mistress had interrupted their evening a little while earlier by calling Tom at home, and he and Daisy had argued about this inside the house while Nick and Jordan sat outside. The "home influence" at the Buchanans is nonexistent. Their "home" is merely a beautiful house because their marriage is one of convenience and infidelity.