This is a holdover from "ye olde tymes" when machines had trouble addressing large hard drives.
As modern machines have lifted that restriction, there is no longer a fixed need for /boot to be separate,
unless you require additional processing of the other partitions, such as encryption or file systems that are not natively recognized by the bootloader.
The modern cases are encryption, new filesystems (such as ext4), LVM, or any esoteric setup that is simply not supported by GRUB.