One of the first things you learn in parenthood is the importance of follow through. In discipline, you must always follow through once a threat is given. With promises, you must always keep them to develop trust in your children.
The same is true in your job and relationships. If trust is the most important currency in the workplace, follow through earns that trust. Did you tell your boss you'd do "that" by tomorrow? Make sure it's done. Did you tell a friend you'd call them later today? Call them.
Many times follow through is the hardest part. Making promises is easy, and completing a task 80% of the way is doable, but it's when you finally deliver on your promise that you've followed through.