The Bell Towers in the land of Kyrat serve several purposes - they are used to call the natives to prayer, they have been co-opted by the government to serve as the towers for the radio masts that beam the official news and propaganda to the People, and they often serve as fast-travel destinations. In addition to that though, they make up one of the Collections in the game.
As a Collection they serve to unlock specific kit - including weapons - so gathering them into the fold so to speak is not just a good idea, it is a necessary one! The following are the locations for the 17 Bell Towers in the game that you need to collect: