Dolphus Raymond is an outlier in the strictly racially divided world of Maycomb. He is a puzzle in the way he violates social norms. He is a wealthy white man from an old family. Jem says of him:
he owns all one side of the riverbank down there, and he’s from a real old family to boot.
All of these factors—whiteness, wealth, and pedigree—are status markers that should propel him to the upper echelon of Maycomb's white society. However, he has violated a strict social taboo by living with a black woman and having children with her. In the racist South of that era, he would not legally be allowed to marry his black girlfriend, but he does live with her as if they are married.
Since it is impossible for the white community to understand why a white man of Dolphus's background would live openly with a black woman, he gives them an easy answer: he cleverly pretends he is an alcoholic. The Maycomb elite can then understand his seemingly aberrant behavior as stemming from his alcoholism. The children find out that the drinking is a ruse during the Tom Robinson trial, when Dolphus offers Dill a sip from his paper bag. The bag does not hide a whiskey bottle as everyone thinks; it contains a bottle of soda.
There's a bit of comedy in this episode in that it is easier for the white community to accept Dolphus as an alcoholic rather than one who could rationally and clearheadedly live with a black woman. It is also another example of the way the children have access to knowledge that is hidden from the adults.