What is the purpose of the outer mitochondrial membrane?
Structure origins

Originally the mitochondrial outer membrane (MOM) was likely the result of a bacteria that could survive in a vesicle formed of the plasma membrane. 



This explains why the plasma membrane and the MOM are structurally similar, however, the proteins that are present in either mean that their functions are very different.The MOM has a specialised translocation complex for example.

Functional significance.

The MOM is actually much more complex than an evolutionary hangover and is essential to modern cells by regulating what that mitochondrial inner membrane is exposed to. The MOM plays this role using porins. Indeed, if the porins of the MOM are disrupted, the entire cell dies (Chipuk et al, 2006). Additionally, the MOM allows a physical association between the ER and the mitochondria called the mitochondria-associated ER membrane (MAM). The MAM is essential for $ {Ca}^{2+} $ transfer.


  The MAM thus integrates signal transduction with metabolic pathways to
  regulate the communication and functional interactions between the ER
  and mitochondrion. (Hayashi et al, 2009)


Answer: The MOM is essential in regulating what the MIM is exposed to, and allows signalling between the ER and mitochondria.