1. It's something different. What I mean by this is that it breaks the monotony of always working on katas and sparring. Anyone that has ever taught children knows the importance of changing things around and working on new things.
2. It forces you to concentrate on your technique. While this should definitely be a focus of empty-handed training, in weapons training this increases ten fold, if for no other reason, out of safety concerns. Imagine what could happen if you're not paying attention while performing a kama routine!
3. It's adaptable. If you look at it, many weapons techniques can be applied to empty-handed fighting as well. Kali/Escrima training is the perfect example.
4. It's practical. Training with weapons also translates to the real world. For example, think of how many everyday items can be used in a similar style as a bo or jo staff: a walking cane, an umbrella, a baseball bat, a random stick lying on the ground, and the list goes on.
5. It makes sense. Contrary to popular belief, there is no such thing as a "fair" fight, especially if your life is in danger. People use all kinds of weapons to get the upper hand, from beer bottles to knives to batons to clubs to guns to chains and on and on. If you know that 9 times out of 10 an assailant is going to be using a weapon, wouldn't it make sense for you to use one too?