Why does Gatsby deliver so many goods and services to Nick's house in The Great Gatsby?
Gatsby knows that the only way to Daisy's heart is through money and objects. He begins his seduction at Nick's, and to do so effectively he must be sure that the stage is set. It would not so to go back to courting Daisy as a commoner. He was once at Nick's level financially and Daisy discarded him because of it. Now, he wants to prove to her that he has all that she desires and can give her all that she wants. He starts his seduction at Nick's, then brings her to his own home where he shows her all of his expensive things (think about the scene in the bedroom with the expensive shirts). Gatsby hopes that, by showing Daisy that he has more than even Tom has in terms of money and possessions, that Daisy will remember that she was once, supposedly, in love with him and will return to him.
I assume that you are talking about what happens in Chapter 5. This is the time that Gatsby wants to use Nick's house to meet up with Daisy. He has not seen her in years.
When this meeting is arranged, Gatsby really wants Nick's house to look nice. This is why he arranges to have a few things done for Nick. He gets a man out to cut the grass, for one thing. And he has a florist deliver a lot of flowers. He is trying to create a nice atmosphere for his first meeting with Daisy since he is trying to get her to love him again.
Gatsby is such a show-off at this place in the novel. Understanding Gatsby was not as well-off at another point in his past relationship with Daisy, this is important for him to prove to her that he's made it.
In this meeting at Nick's, he knows Nick's house is very humble in comparison to his and Daisy wants the flash. She's all about her money and if he's going to get in the game with her, he has to prove that he has it. Improving Nick's environment lays the groundwork for heading over to Gatsby's house later in the chapter.