The ATP cycle allows the cells to have a constant supply of ATP in order for them to keep performing their functions. ATP is needed mostly by energy-requiring reactions. Some reactions don't need the input of ATP to take place because they are energy-releasing reactions.
So, when there is a release of free energy by energy-releasing reactions, the free energy is used by the energy-consuming reactions. However, ATP is a relatively simple molecule, meaning it won't be enough to power all energy-consuming reactions. That said, the ATP cycle is preserved through the hydrolysis of ATP to give ADP and Pi (an inorganic phosphate group).
This is a reversible reaction, meaning cells can easily get ATP by reversing the reaction. To understand this better, think of ADP as an uncharged battery that can be used for cellular activities by recharging it once the charged battery (ATP) has been used up.