Arrest gives them legal authority to search the person of the accused and find evidence of other crime. Perhaps they will find something that leads to a felony and authorization to hold. For instance, you probably wouldn't want to release someone who was heavily intoxicated in public while in possession of a deadly weapon.

To be clear, I'm not saying I'm thrilled about prosecuting people for many of the crimes for which the contraband would constitute, but the advantages of arresting the perp even if they'll immediately be released is obvious.