From the docs of the
Flushes the output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.
The buffering is mainly done to improve the I/O performance. More on this can be read from this article: Tuning Java I/O Performance.