It is the use of disguises that provides those wearing them important information and intrigue.
In the case involving the King of Bohemia and Irene Adler, an actress, the King is about to be married, but is worried about a compromising photograph of himself with Miss Adler; this photograph is in the possession of Miss Adler, who refuses to relinquish it. Disguised as the agent of the king, the King himself arrives wearing fur-trimmed boots that reach past his calves that "completed the impression of barbaric opulence which was suggested by his whole appearance." He wears also a black vizard mask. But Holmes quickly perceives that the man poses as someone other than he is. After he reveals himself, he asks Holmes to retrieve this photograph for him.
So, Holmes himself goes into disguises in order to obtain information. Watson notes that Holmes could have become a fine actor because
It was not merely that Holmes changed his costume. His expression, his manner, his very soul seemed to vary with every fresh part that he assumed.
He first poses as a a slovenly groomsman in order to hear the gossip of the stables and the cabbies. From them he learns that Miss Adler is about to be married. In fact, he is solicited to be a witness to this wedding. Then, because the newlyweds are about to leave town, the urgency of finding the photograph is heightened.
Holmes's next disguise is that of a clergyman who mingles with the crowd outside Irene Adler's home, jostled by this crowd, the clergyman is knocked to the ground and bloodied. Concerned about him, the actress has him brought inside. In the meantime, Watson throws a smoke bomb into the house and cries of "Fire" resound, causing the occupants to flee the house. But first, taking advantage of the situation, Holmes watches where Irene rushes to, knowing that she would wish to retrieve her valuable photograph. He discovers, then, her hiding place.
The next morning the King accompanies Holmes to the Adler house; however, a servant informs them that the newlyweds have departed on their honeymoon. Nevertheless, she invites the men to enter the drawing-room where Holmes finds in the hiding spot another photograph of the actress along with a note addressed to him. In it Miss Adler informs Holmes that as an actress, she, too, could assume other identities.
Disguised herself, then, she has turned the tables on Holmes and followed the King and Holmes back to his dwelling, as her letter, with another single photograph of her, reveals. Her letter states that she will cause no more problems for the King, and Holmes is so impressed with her that he asks the King to give him the photograph as his reward.
Thus, the use of disguises adds interests, provides information, moves the plot forward, and even adds a little intrigue to the narrative of "A Scandal in Bohemia."