As the name suggests, hepatocyte nuclear factors are expressed predominantly in the liver.  However HNFs are also expressed and play important roles in a number of other tissues so that the name hepatocyte nuclear factor is somewhat misleading.  Nevertheless, the liver is the only tissue in which a significant number of different HNFs are expressed at the same time.  In addition, there are a number of genes which contain multiple promoter and enhancer regions each regulated by a different HNF.  Furthermore, efficient expression of these genes require synergistic activation by multiple HNFs.  Hence hepatocyte nuclear factors function to ensure liver specific expression of certain genes.
As is the case with many transcription factors, HNFs regulate the expression of a wide variety of target genes and therefore functions.  These functions (and especially functions involving the liver) include development and metabolic homeostasis of the organism.  For example, HNFs  influence expression of the insulin gene as well as genes involved in glucose transport and metabolism.  In embryo development, HNF4α is thought to have an important role in the development of the liver, kidney, and intestines.