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Separating /var matters because logs and container images grow without bound and a full root filesystem prevents the system from functioning at all. Reserving about a gigabyte for the EFI system partition and /boot leaves room for several kernel versions. Using LVM for the rest means these boundaries can be adjusted later rather than being permanent.
Partition plan
usable = total - swap - boot reservation; root = usable x root share; /var = usable x var share; remainder to /home and data
So runaway logs or container storage cannot fill the root filesystem. A full root breaks logging, package management and often SSH login.
LVM gives separate logical volumes that can be grown, which is the best of both worlds: isolation now and flexibility later.