Shug is coming to town to sing. Mr. ____ prepares himself to meet her (he has not seen her for an undefined but significant period of time). He shaves and asks Celie to wash and iron his clothes, in order to make a good impression on her. Shug's arrival has dramatically improved Mr. ____'s mood, and he is nicer to Celie as a result.
One way that Shug helps Celie, without even knowing it, is by simply arriving in town. Shug's very presence is enough to make Mr. ____ happy, and when Mr. ____ is happy, he tends to take out his aggression on Celie less frequently.
Celie sees a copy of the bill advertising Shug's show. Celie wants to go: not to dance or drink, but just for a chance to see Shug in the flesh.
This is the second time in the novel that Celie has become entranced by a picture of Shug in her singing costume. The idea of Shug is almost as powerful as Shug in the flesh.