When rewards can be measured, the importance of the project can be judged from looking at the reward. The greater the reward, the more important the project is.
The problem with this thinking is that some rewards are difficult to measure (compare the number of people who attended your recital with a tear in an elderly woman's eye afterwards). If the reward is difficult to measure, many people tend to stay away from such a project.
Another alternative is choosing to do things that matter simply because it's the right thing to do. Because you can.