What is the purpose of exporting all of sysroot?

[Makekihe]

I'm trying to modify this repo, so that I can use it for true cross compilation, as an alternative to the QEmu emulator. I'm doing it based on the work on the emersonknapp/cross-compile branch, where there is some work-in-progress. I've stumbled upon something which I don't really understand on the branch, and I'm hoping for some clarification.

What is the purpose of exporting all of the sysroot?
Where is this used at a later stage?

The exploring using fs.extractall() takes +1 hour for me to do of my docker file (~70k files, 2.4 GB), so I'm interested in doing this more effectively. Do you have any idea why this takes so long, and if any alternative could speed up the process?

Branch: emersonknapp/cross-compile branch
File: sysroot_creator.py
Code:
"""
logger.info('Successfully created sysroot docker image: %s', image_tag)
logger.info('Exporting sysroot')
fs = docker_client.export_image_filesystem(image_tag)
logger.info('Extracting sysroot to destination')
try:
shutil.rmtree(str(sysroot_destination))
except FileNotFoundError:
pass
fs.extractall(sysroot_destination)
fs.close()
"""