Red Hat Enterprise Linux 7.2

ARMv8 is one standard, but it only covers the CPU architecture itself, not any of the "peripherals" (like SATA, Ethernet, PCIe, serial ports etc). It's thus very limited in what it can guarantee - an ARMv8 implementation might have RAM at physical addresses 0 to 1G, or at physical addresses 64G to 65G, it's not guaranteed to have discoverable peripherals (you have to Just Know what's on the motherboard, and what physical addresses it's at), if it does have discoverable peripherals you may first have to go through a vendor-specific setup process to turn on the discoverable bus (e.g. a magic sequence to enable PCIe).

SBSA and SBBR are ARM's attempts to fill this gap for ARMv8 servers; just as it's unusual to encounter x86 systems that don't have either an IBM PC BIOS compatible or UEFI compatible platform on top that lets a generic OS boot, so ARM hope that ARMv8 servers will implement SBSA and SBBR, and it'll be unusual to encounter an ARMv8 server that needs the OS to know specifics of the machine it's booting on.

Note that ARM's in a difficult position here - its traditional customers don't want to run generic OSes on their hardware, they're happy hard-coding every detail of the MCU or SoC they run on into the OS. The advantage is an OS that boots and runs quickly on their hardware, with no platform firmware in the way; the disadvantage is that the OS and the hardware are tightly coupled, and can't be used in isolation.