I see that many missions have a challenge associated with them, in order to get 100% synchronization. What benefit(s) do I get for achieving 100% synchronization? Some of the challenges are kind of hard, and there seem to be a million of them, so I want to understand if it is worth it.
There are a couple of achievements associated with synchronization:
Each sequence also has an associated cheat that is unlocked for 100% sync, which can be used in single player. You can only use cheats while replaying a mission, so they're really more for replay value than for making your life easier.
There are also a series of "repressed memories" that you unlock as you increase your "overall" synchronization, and completing missions with 100% sync is a good way to increase this total. However, the last one of these is unlocked at 75% total synchronization, so 100%'ing the entire game isn't required. These memories are from before Ezio became an assassin, and center around his relationship with Cristina Vespucci.