The simple answer is that Calpurnia adds to all the forebodings about her husband Caesar’s safety, while Portia helps to bring out the emotional torment her husband Brutus is experiencing because of his involvement in the plot to assassinate Caesar. However, it would appear that Shakespeare had another reason for including these two female characters in his play. Without them, the play would have the impression of a bunch of men dressed up to represent ancient Romans and strictly involved in one-sided male talk and male business. With the female characters, Shakespeare tries to achieve what is called “orchestration,” i.e., some variation in the appearances, voices, costumes, motivations, dialogue of the cast.
It will be observed that in most of Shakespeare’s plays, notably in his major ones, he “orchestrates” his cast. In Hamlet, for example, he has young men and a young woman (Ophelia), middle-aged Claudius and Gertrude, and even the elderly Polonius. There are important female roles in all his major plays. In Macbeth he has Lady Macbeth as well as three witches. In Othello he has Desdemona , Emilia, and Bianca. In King Lear he has three important female characters in Cordelia, Goneril, and Regan. In Antony and Cleopatra he has Cleopatra and some of her female attendants.
These female characters, of course, were played by female impersonators. No doubt, Shakespeare had young actors in his company who specialized in such roles and he made a point of using them in every production. (The boy who played Lucius in the fourth act of Julius Caesar may have also played Calpurnia or Portia—if not both women—earlier in the same performance.)
The term “orchestrate” derives from opera. In operatic works there are usually roles for a soprano, a tenor, a contralto, a baritone, in order to provide variety and contrast. Variety and contrast are important considerations in all artistic works. The same principle is usually followed in plays. A modern example would be A Streetcar Named Desire, in which there are two major roles for men and two for women—Stanley Kowalski and Mitch, on the one hand, and Blanche and Stella on the other.
Lajos Egri devotes a chapter to "orchestration of characters" in his excellent and widely read book, The Art of Dramatic Writing.