how to extract frames without save a video?

the document for extractMultipleFrames is not clear, since we add the folder to method extractMultipleFrames, why we need to add another path to save method? the folder is the place to save all frames, but specify it there doesn't work as expect, we have to extract frames when transcode a video into another format, this is not what we need.

$video
    ->filters()
    ->extractMultipleFrames(FFMpeg\Filters\Video\ExtractMultipleFramesFilter::FRAMERATE_EVERY_10SEC, '/path/to/destination/folder/')
    ->synchronize();

$video
    ->save(new FFMpeg\Format\Video\X264(), '/path/to/new/file');