As for why: A power loss will likely only affect filesystems being written to. So less risk of ending up with an unbootable system if / and /usr are not being written to.
Even moreso if they are mapped read-only.
I don't know about now, but I've seen people only enable softdep mount option for FSs that need some extra speed, and not on / and /usr. (article does this for /home. look at the listed mount options)
It's also nice to mount /tmp and /home with "nosuid" and "nodev". Some file systems even "noexec".
Edit:
Note that I'm not advocating for it. I'm merely listing reasons one could have.
As for why: A power loss will likely only affect filesystems being written to. So less risk of ending up with an unbootable system if / and /usr are not being written to.
Even moreso if they are mapped read-only.
I don't know about now, but I've seen people only enable softdep mount option for FSs that need some extra speed, and not on / and /usr. (article does this for /home. look at the listed mount options)
It's also nice to mount /tmp and /home with "nosuid" and "nodev". Some file systems even "noexec".
Edit:
Note that I'm not advocating for it. I'm merely listing reasons one could have.