There are many benefits to rewarding people for what is already expected. You have to know exactly what to reward them for, and they have to know what you are rewarding them for. Otherwise, it can be a negative outcome and it will continue because it is being rewarded.
If you reward someone, especially a child, for doing something they are expected to do then they will be more likely to do it again. This would cause a positive reaction in both the giver and the receiver. Rewarding someone almost always makes them happy and they feel appreciated. When someone feels appreciated then they will most likely be happy. There are some people who don't like to be rewarded for things and there are others who expect it. In either of these cases this could cause a negative outcome.
I believe that if you reward people every time they do something good then they will expect it every time.
If you do not reward them every time then they may only do it occasionally. In this case it could be a negative outcome. If they do not know exactly what you are rewarding them for then they may not know exactly what to keep doing that is a good behavior. If you have a child and you reward them for using the bathroom then you have to tell them what you are rewarding them for and reward them as soon as possible. Children have a short memory span and they may forget what you are rewarding them for if you wait too long. This goes for punishments as well as rewards.