Historically the /usr directory was on an other partition and got mounted to / during the boot process. The boot process made use of /bin, /sbin, /lib and /lib64.

Modern approaches do not separate required from not-required directories for the boot process anymore. Everything is in /usr that has to be available during the boot process. For backward compatibility symlinks are added in the /bin, /sbin, /lib and /lib64 directories pointing to /usr/bin, /usr/sbin, /usr/lib and /usr/lib64

In the root directory the following directories should be present and used in the proposed way:

/bin

System binaries and binaries required to boot

/boot

Boot loader and kernel

/dev

Device files

/etc

Configuration files and boot and runlevel scripts

/home

All regular users

/lib

Shared libraries. In/lib/modules are the kernel modules

/lost+found

Stuff that fsck found

/media

Mounting point for removable media devices

/mnt

Mounting points for temporary file systems on other media.

/opt

Additional program packages. Alternative /usr/local

/proc

Information about running processes

/root

The home directory of the user root

/sbin

System administration binaries and binaries required to boot

/sys

The kernel exports information about devices and drivers to this directory.

/tmp

temporary data mounted as RAM disk using tmpfs. Alternative/var/tmp be aware that this directory gets wiped off when booting the system.

/usr

Static read only user data (/var should hold the dynamic), kernel source

/var

Dynamic data log files, printer spool, mail

To observe directories it might be handy to emerge app-text/tree and use it as tree /var/log.