There are several possible answers to your question. In the first instance, Caesar is asking a rhetorical question of his erstwhile friend when he addresses him by saying, "Et tu, Brute?" Obviously, he does not expect Brutus to answer him or explain himself; on the contrary, Caesar already knows that Brutus has also turned against him.

Another thing that makes this line particularly notable is the fact that Shakespeare chooses to have Caesar say it in Latin. Obviously, these characters would have originally spoken in Latin, but in the context of this play, the use of Latin is an example of aureate diction--the use of a classical language to highlight something as being of particular significance. "Et tu, Brute?" is a very formal phrasing that uses Latin to render the moment more ceremonial and somber than it might otherwise have seemed.

This sense of ceremony is perpetuated in Caesar's use of illeism, namely, his reference to himself in the third person. In saying "then fall, Caesar," he creates the impression that "Caesar" refers to more than simply himself as a man; it also refers to the symbol of Caesar, the would-be king, and his power over Rome.

Caesar is speaking of himself in the third person in this quotation. To speak of yourself in the third person means to use your own name when referring to yourself, instead of saying "I."

If Caesar were speaking normally here, he would have said "Et tu, Brute?--"then I fall!"

To speak of himself in the third person adds a little more drama to the scene. Caesar is giving in to the idea that he is going to die, that he will cease to be. Now he is just about to become a historical figure, so it seems more fitting than it usually might to refer to himself this way.

Keep in mind also that Caesar is a title rather than a proper name. There were a number of Roman "Caesars." That makes it sound like he's saying something along the lines of, "You have killed your Caesar."

Something is exemplified when one uses it as a typical example or illustration. In this excerpt from Act 3, Scene 1, there are, in fact, two powerful examples of rhetoric which Caesar uses. The first and most obvious is the rhetorical question, "Et tu, Brute?" English equivalents for the question could be, "And you, Brutus?" or, "You too, Brutus?" or, "Even you, Brutus?"

A rhetorical question is one that does not require an answer. The question is asked to make a point or to express a sentiment. In this instance, Caesar asks the question after Brutus stabs him. The general is expressing his utter shock and dismay that one such as Brutus should betray him. 

The second example of rhetoric is the illeism used by Caesar when he exclaims, "Then fall Caesar!" Illeism is the act of referring to oneself in the third person. Caesar refers to himself in this manner from the start of the play. Act 2, Scene 2 depicts another example when he says the following:

Yet Caesar shall go forth; for these predictions
Are to the world in general as to Caesar.

Caesar's use of illeism reflects his belief that he is a near-deity. It is an expression of his arrogance and feelings of superiority. He is so confident about his power and position that he refuses to believe the predictions and warnings of others, such as the soothsayer, his wife, Artemidorus, and the priest, to be wary. 

Shakespeare might also have decided to use this rhetorical device to copy the style in which the real Julius Caesar recorded his exploits in the Gallic Wars when he wrote his Commentarii de Bello Gallico. In this case, Caesar provides a first-hand account of his experiences in a third person narrative. 

In Act III, Scene 1 of Shakespeare's Julius Caesar as the conspirators approach and Caesar asks why Brutus kneels uselessly, Casca exclaims, "Speak hands for me!" and he and the others stab Caesar.  Caesar sees his friend Brutus and asks the rhetorical question, "Et tu Brute?" which is Latin for "And, you, Brutus?"

A rhetorical question differs from another type of question used in rhetoric, hypophora, in that it is asked without the expectation of a response.  Caesar, completely surprised by the conspirators, is even more amazed to see Brutus as part of them.  The reader will remember that Caesar has mentioned to Marc Antony in Act I that he is wary of the appearance of Cassius who has a "lean and hungry look."  However, Caesar has never entertained the idea that Brutus may wish to assassinate him; instead, Caesar believes Brutus an ally and friend.  His rhetorical question, therefore, expresses this surprise and disbelief, as well as it expresses a disappointment in the noble Brutus.  It is, indeed, more of an emotional expression that a question per se.