The AArch64 support includes a larger register-file, enhanced addressing range and virtualization awareness.
ARM's OEMs are using UEFI to boot ARM-based systems. It is applicable to a wide range of segments since UEFI specifies a framework which can be extended and reduced as required. And it generally helps standardize the boot procedure from power-up to booting an Operating System.
UEFI also saves time and money. UEFI specification allows the ARM partnership to write one boot loader per platform which in turn saves costs in boot loader development and engineering.
Finally, it provides an environment for manufacturing tests.