Why people observe fast?

A group of people observe fasting without having particular aim and intention which could be the basis of correctness of their deeds and accordingly could prevent their deeds from becoming nullified.

For example, some might observe fasting, because of fear from the people, or for the sake of their material and worldly benefits, or may observe fasting because of prevailing habits among Muslims.
 

Some observe fasting in order to remain immune from people’s persecution as well as not to remain deprived from material gains and advantages obtained through intermingling with the community, some observe fast in order to remain immune from peoples persecution, as well as not to remain deprived from material advantages obtained through social intermingling; in order to remain immune from Hell’s punishments as well as not to be deprived from the bounties of Paradise.

Another group observe fasting only for the sake of immunity from Hell’s punishment, or for utilization of Heavenly bounties, or for the sake of both.

Another group observe fasting for the sake of immunity from Hell’s punishment as well as to be blessed with Heavenly bounties, but in addition to that, also to seek Allah’s pleasure and nearness.

In the end there is a group who observe fasting and does not have any other intention and aim except Allah’s pleasure and nearness.

They observe fast only in order to become closer to Allah (Glory and Greatness be to Him), and to attain the position of His consent.

Reference:- Spritual Journey of the Mystics(Suluk e Irfaan) - By Late Haj Mirza Javad Agha Malaki Tabrizi(R.A.)