Matthew 6:16 Moreover, when you fast, do not be like the hypocrites, with a sad countenance. For they disfigure their faces that they may appear to men to be fasting. Assuredly, I say to you, they have their reward. 17 But you, when you fast, anoint your head and wash your face, 18 so that you do not appear to men to be fasting, but to your Father who is in the secret place; and your Father who sees in secret will reward you openly.
Fasting, is a religious duty, if you will. Its a means to starve our carnal desires and set priorities on spiritual matters. Those in this time wanted others around them to know just how devouted they were. All external. The LORD says, do these things in such a way, its just you and the Father. Then, He will reward us openly what we seek Him privately for. Only a living God can fulfill this promise. What a way to really be grounded in our faith as well. I dont want mans reward, I want the LORD to say, "good and faithful servant".