It seems that it is a primarily time-management strategy. From the Oyahocon Cosplay Competition rules (bold text mine):
The music and/or prerecorded dialogue must be ready to go as soon as it is put the CD player (this means the participant must edit the music and/or prerecorded dialogue before the convention and put it on a CD). Audio file must be exactly three (3) minutes or less. There are NO exceptions to the rule. If the audio file is longer than three (3) minutes, it will be stopped when time is reached (even if it cuts the performance). Please have files recorded exactly as you wish them to be played.
From Anime Midwest competition rules:
The pre-recorded music/dialogue submitted to the Masquerade staff must be 2 minutes and 30 seconds or less. There will be no exceptions to the "2 Minute" rule, unless you request permission prior to the convention. Any entry over the 2:30 mark will be stopped at that point.
From Anime-Expo Performance competition rules:
Adhere to the 2 minute time limit.
MC and video introductions will not count towards the performance time.
Audio: Any sound necessary to express your performance (i.e. music, pre-recorded dialogue, sound effects).
Must be comparable to a PG rating– no profanity or offensive language. Bleeps or other forms of censoring are acceptable.
Should not exceed 2 minutes.
Must be MP3 format.
Must have minimum 192 bitrate.
Try to avoid audio quality defects (audible distortion, overdriven levels, low sample-rate/bitrate).