According to the definition of endl, it is used to insert a new-line character and flush the stream. And I remember that if a new line is inserted, then the buffer will be flushed automatically. If so, why do endl still need to flushes the stream after inserting a new line.
if a new line is inserted, then the buffer will be flushed automatically
Not necessarily, and not for all streams. It's common for std::cout (and the standard output stream it sends data to) to be line-buffered, but not universal. Some implementations, for example, only line-buffer stdout if output is going to a terminal, and fully-buffer it otherwise (if output is redirected to a pipe, for example).
And that's just for standard output. Other streams like file I/O are almost always fully-buffered and the buffer will only be flushed when it either fills up or its flushed explicitly.