It's actually not 15 practices but it still rewards you the later the bowl game is.
"NCAA rules allow bowl teams to practice or hold other athletic activities up to four hours per day or 20 hours per week between the end of the regular season and a bowl game. While coaches often refer to the "15 extra practices" that come with a bowl invitation, there is no specific limit to the number of practices a team can hold during bowl preparation."
If you don't make a bowl:
"Teams that don't reach bowl games are limited to mandatory weight training, conditioning and review of game film between the end of the regular season and Jan. 1. They get a maximum of eight hours per week for those activities, including no more than two hours per week of film viewing."