Approximately half of proteins with experimentally determined structures interact with other copies of themselves and assemble into homomeric complexes. A number of functional benefits to homomerization have been proposed, and it is often taken for granted that it has been selected for evolutionarily. However, there has been little systematic analysis of the relationship between homomer structure and function. Here, utilizing the large numbers of structures and functional annotations now available, we have investigated how proteins that assemble into different types of homomers are associated with different biological functions. We observe that homomers from different symmetry groups are significantly enriched in distinct functions, and we can often provide simple physical and geometrical explanations for these associations in regards to substrate recognition or physical environment. One of the strongest associations is in dihedral complexes and we suggest that much of this can be related to allosteric regulation. We provide a physical explanation for why allostery is related to dihedral complexes: it allows for efficient propagation of conformational changes across isologous (i.e. symmetric) interfaces. Overall we demonstrate clear relationships between protein function and homomer symmetry that have important implications for understanding protein evolution, as well as for predicting protein function and quaternary structure.