As a Shakespearean comedy, the main significance and purpose of disguise and false communication in Twelfth Night is to create irony, especially dramatic and situational irony. For example, Duke Orsino hires Cesario (Viola disguised as a man) to woo Olivia for him, and Cesario ends up falling in love with the Duke who, unknowingly, gives this smitten woman personal information one would only give a close friend and confidante, further "lighting her fire". As far as false communication, Malvolio is led to believe through a cryptic letter that Olivia is in love with him. This miscommunication creates irony because Malovolio is so self-centered and vain that he automatically assumes that someone of Olivia's social standing and beauty would be in love with him when she actually has no interest in him whatsoever. In addition, a combination of disguise and false communication help to develop indirect characterization and show us that love may be foolish and transitory and may cause one to be easily misguided.
The disguises and false communication illustrate that one's appearance is only an illusion. It can be symbolic in that this lets us explore the lack of reality that can be accomplished by keeping up appearances or showing the public what you want them to believe you represent. Although it is a comic perspective, the issues it presents allow for serious analysis.