A mouse with higher precision will allow you to optimize your preference. Little movement of the hand can be converted into a fast motion on screen, which can be a benefit in shooters for example. They also have additional buttons to map keys to, making certain things easier. And are ergonomic.
As for the keyboard, they generally also have additional keys for key mapping. The keys are also of higher quality to last through hard presses and so on. Some say they have faster input and so on, but in reality that's all negligible.
Personally, I only use gaming mice, and no gaming keyboard.