What is the purpose of a double pole breaker for separate 15A circuits?
I'm guessing the circuits are sharing a neutral, and so are considered a multiwire branch circuit.  


  Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a
  grounded conductor that has equal voltage between it and each
  ungrounded conductor of the circuit and that is connected to the
  neutral or grounded conductor of the system.


In the 2008 version of the National Electrical Code (NEC), they introduced sub section (b) to section 210.4.  This sub section covers the disconnecting means of a multiwire branch circuit, and requires a simultaneous disconnect of all ungrounded conductors.


  NEC 2008
  210.4 Multiwire Branch Circuits.
  (B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all
  ungrounded conductors at the point where the branch circuit
  originates.


This subsection can be satisfied by using a multipole breaker, which provides both simultaneous trip and simultaneous disconnect. The subsection can also be satisfied by simply attaching handle ties between multiple single pole breakers, which would provide simultaneous disconnect (and may or may not provide simultaneous trip).