rationale for default ZFS datasets

If at installation time one chooses automatic guided ZFS install, the
result is a set of datasets as below. Presumably this default set is
meant to cater for the most common use cases. What is not clear to me
is the rationale for that particular shape. It may be explained
somewhere on the internet. If anybody knows where please send me the
link. Otherwise I will ask a few questions.

/var and /usr both merit a dataset of their own, and under them there
are a few other nested datasets. What is not clear to me is why only
these particular folders merit their own datasets, likewise with /tmp.
Perhaps it is so that one has the ability to do snapshots?

zroot could be left unmounted, so why is it mounted at /zroot?

why zroot/ROOT needs a dataset of its own, and why is it unmounted?

what is the purpose of zroot/ROOT/default?

Lots of questions! As I said, if this is explained somewhere please
let me know where.

root@fbsd-vm:~ # zfs list
NAME                 USED  AVAIL     REFER  MOUNTPOINT
zroot               4.38G  32.0G       96K  /zroot
zroot/ROOT          3.40G  32.0G       96K  none
zroot/ROOT/default  3.40G  32.0G     3.40G  /
zroot/tmp           1001M  32.0G     1001M  /tmp
zroot/usr            508K  32.0G       96K  /usr
zroot/usr/home       220K  32.0G      148K  /usr/home
zroot/usr/ports       96K  32.0G       96K  /usr/ports
zroot/usr/src         96K  32.0G       96K  /usr/src
zroot/var           1.02M  32.0G       96K  /var
zroot/var/audit       96K  32.0G       96K  /var/audit
zroot/var/crash       96K  32.0G       96K  /var/crash
zroot/var/log        540K  32.0G      540K  /var/log
zroot/var/mail       124K  32.0G      124K  /var/mail
zroot/var/tmp         96K  32.0G       96K  /var/tmp