The cellular process of breaking down large molecules into smaller ones is defined as

The cellular process of breaking down large molecules into smaller ones is defined as catabolism.

Catabolism together with anabolism is part of the metabolism-set of all chemical reactions within an organism. Purposes of metabolic processes are to convert food energy into energy that is required for cellular processes, to synthesize the building blocks for proteins, lipids, nucleic acids, and some carbohydrates (macromolecules) and to eliminate nitrogenous wastes.

Catabolic processes (breaking down of macromolecules) are necessary for energy release (usually in the form of ATP) which is then used in anabolism-synthesis of compounds that cell needs.