Fire ants will eat other ants. Army ants will also eat other ants and actually seem to specialize in raiding the broods of other ant colonies. In fact fire ants will eat almost any kind of insect they catch. Ants (Formicidae) are a very broad genus. Asking what do ants eat is like asking what do people eat. It depends on which ants.
Ants live underground in giant colonies. When they build tunnels, the tunnels sometimes collapse. This moves the dirt around and brings rich soil to deep parts of the ground. Ants are also part of the cycle of life. They eat other small insects and get eaten by animals such as anteaters. They also eat wood.