Proper folding of proteins is essential for their biological activity. In general, the functional conformation of a protein is the conformation with lowest energy. This means that if an unfolded protein is allowed to reach equilibrium, it should assemble automatically into its native, functioning folded state. Why then is there a need for molecular chaperones and chaperonins in cells?

Molecular chaperones bind to nascent and folding or misfolded proteins (through improperly exposed hydrophobic regions) and stabilize the polypeptide so it can fold into its native state
They have the ability to prevent non-specific aggregation by binding to non-native proteins
They play an important role in protecting cells from being damaged under environmental stress, such as extreme heat, poisoning, or mental stress
Most common chaperones are heat shock proteins which are also called stress proteins
The intracellular folding environment for proteins is chaotic because of the presence of other biomolecules, folding proteins, improper or fluctuating pH, heat, and other denaturants
Chaperonins provide actual chambers for misfolded proteins to properly refold, segregating the protein from the chaotic cellular environment.
The main key function of chaperonins is to assist in the folding of large protein molecules