A Packet is a category within a category. You can use them to limit the amount of airplay some songs get within their ‘master’ category. You can also use them to increase the airplay of a song, to have it scheduled twice as often as other songs in the same category. Or, you might want to rotate two versions of the same song without doubling up on the number of times the song itself gets scheduled.
This video shows how have M1 schedule a long version of a song at night and the shorter version of the song during the daytime. And, it demonstrates how to get one song to play twice as often as the other songs in the same category.