Biology depends on complex interactions between organisms, organs, and molecules, depending on your perspective. The biochemistry of supramolecular complexes emphasizes interactions between macromolecules that are associated by non-covalent interactions.
Consider these examples: A simple virus is a dynamic machine comprised of a piece of RNA contained in a self-assembling multi-protein cage; how does it regulate infection? In cells, how does a membrane protein like a GPCR transduce a signal to its associated G-protein? How can folding a protein be modulated by ligands and a specific, fluctuating environment. Interaction between molecules is essential to building large, biologically active complexes from small components.