Why do I have to send STREAM_BLOCKED?

[ekr]

   If a sender does not receive a MAX_DATA or MAX_STREAM_DATA frame when
   it has run out of flow control credit, the sender will be blocked and
   MUST send a BLOCKED or STREAM_BLOCKED frame.  These frames are
   expected to be useful for debugging at the receiver; they do not
   require any other action.  A receiver SHOULD NOT wait for a BLOCKED
   or STREAM_BLOCKED frame before sending MAX_DATA or MAX_STREAM_DATA,
   since doing so will mean that a sender is unable to send for an
   entire round trip.

If this is just for debugging, why is it required?

[martinthomson]

I have the same question in #65.