Objection 1. It would seem unnecessary for the accusation to be made in writing. For writing was devised as an aid to the human memory of the past. But an accusation is made in the present. Therefore the accusation needs not to be made in writing.
Objection 2. Further, it is laid down (Decret. II, qu. viii, can. Per scripta) that "no man may accuse or be accused in his absence." Now writing seems to be useful in the fact that it is a means of notifying something to one who is absent, as Augustine declares (De Trin. x, 1). Therefore the accusation need not be in writing: and all the more that the canon declares that "no accusation in writing should be accepted."
Objection 3. Further, a man's crime is made known by denunciation, even as by accusation. Now writing is unnecessary in denunciation. Therefore it is seemingly unnecessary in accusation.
On the contrary, It is laid down (Decret. II, qu. viii, can. Accusatorum) that "the role of accuser must never be sanctioned without the accusation be in writing."
I answer that, As stated above (Question 67, Article 3), when the process in a criminal case goes by way of accusation, the accuser is in the position of a party, so that the judge stands between the accuser and the accused for the purpose of the trial of justice, wherein it behooves one to proceed on certainties, as far as possible. Since however verbal utterances are apt to escape one's memory, the judge would be unable to know for certain what had been said and with what qualifications, when he comes to pronounce sentence, unless it were drawn up in writing. Hence it has with reason been established that the accusation, as well as other parts of the judicial procedure, should be put into writing.
Reply to Objection 1. Words are so many and so various that it is difficult to remember each one. A proof of this is the fact that if a number of people who have heard the same words be asked what was said, they will not agree in repeating them, even after a short time. And since a slight difference of words changes the sense, even though the judge's sentence may have to be pronounced soon afterwards, the certainty of judgment requires that the accusation be drawn up in writing.
Reply to Objection 2. Writing is needed not only on account of the absence of the person who has something to notify, or of the person to whom something is notified, but also on account of the delay of time as stated above (ad 1). Hence when the canon says, "Let no accusation be accepted in writing" it refers to the sending of an accusation by one who is absent: but it does not exclude the necessity of writing when the accuser is present.
Reply to Objection 3. The denouncer does not bind himself to give proofs: wherefore he is not punished if he is unable to prove. For this reason writing is unnecessary in a denunciation: and it suffices that the denunciation be made verbally to the Church, who will proceed, in virtue of her office, to the correction of the brother.
Source: Summa Theologica (New Advent)