FreeNas offers to install itself on a disk, and only then create a Raid, how true is this?

Question

linux

I want to make a home NAS, freenas suggests that you first install yourself on a hard one, and only then create a raid.
I don't see the logic, why not make a raid first, and only then install OS on it?
What is the reason for such an offer from freenas?

Answer 1

Maybe because this system uses too "adult" technology?
I (not FreeNAS, but the closest relative NAS4Free) has 2 memory cards in the mirror (you can also put it on a USB flash drive).
And RAID with data - separate 5 disks (RAID-Z2)

Answer 2

And it just won't work the other way.
This is the only possible option.