“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. But you, when you fast, anoint your head and wash your face, 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.” – Matthew 6:16-18 NKJV
The point of fasting in private isn’t about the reward, it’s about the audience. In private moments, the audience is narrowed down to the Lord. Jesus prioritized time for conversation with the Father; and with it, He often denied the flesh through fasting. He was referencing food, but there can be other types of fasting as well.
The flesh has many forms of appetites and denying them involves the will. Other people’s awareness is inconsequential (unless He leads you to tell them), because the main benefit is the investment itself. Following through on a private commitment to the Lord can produce eternal results; and a hidden discipline can affect many visible things.