I'm trying to follow this guide and one of the steps is to create users with home dirs and even to preserve /home. What't the reason behind this?
It is an example to use the file sharing.
we'll create two (2) new users on the LEDE system for file sharing purposes.
You can omit that if you don't need it.
Create as many or as little as you like
File sharing dir is set to /mnt/sdb1/Backups, that doesn't explain why /home is created and preserved in this tutorial.
If you are going to login as root to share the files, then you don't need the users.
I'm not asking why users are created. I wonder why would you create home directories for them. BTW web UI sets home to /tmp, that makes some sense but I would skip home at all if nothing is supposed to be stored there.
... till the next boot, when it will be gone.
But what are you going to store there? Actually the answer should be "nothing" and in this case there's zero sense in creating home directories outside tmp and preserving them. I didn't manage to find any files there.