Ewwww, seriously, why?
To terrify the opposing army and make them go from "yes, Critical Hit!" to "OH FUCK RUN!"
Bear in mind that Romance of the Three Kingdoms is a highly romanticized (heh) version of the historical events surrounding these empires. Exhibit A being the unstoppable force of badassery that was Lu Bu, he was not as badass as the book says he is, he got an upgrade from "above average warrior" to "slayer of armies". Same with people like Guan Yu (who actually became a kind of Chinese 'patron saint').