As of Oracle Database 10g release 10.2, if Streams functionality is used, but the STREAMS_POOL_SIZE initialization parameter is not defined, then the size of the streams pool automatically defaults to 10% of size of the shared pool.
When the streams pool is created, the required SGA memory is taken from memory allocated to the buffer cache, reducing the size of the cache to less than what was specified by the DB_CACHE_SIZE initialization parameter. This means that if the buffer cache was configured with only the minimal required SGA, then Data Pump operations may not work properly.
A stream pool is required. The stream pool is used to cache the queue messages used by the stream process to move and copy data. expdp is an api provided internally by oracle.