Apart from enabling Viola to protect herself in a strange land, what are the dramatic advantages of a disguised character?
In Elizabethian plays (Shakespearian comedies especially), the ideas of twins separated and gender switching were considered hilarious and poignant. Audiences would follow the character and laugh at others' inability to tell that the male/female character is, in fact, a member of the opposite sex. Interestingly enough, most, if not all, of the parts of plays were played by men so Viola would be a male actor, playing the role of Viola, who is disguised as a man, Cesario. The dramatic advantage would be that the character of Cesario could be even more convincing as a man, therefore, spiriting the play throughout.