There’s a large element of “because I can” to this, just to explore how stupid the scope of systemd is as a suite.
There’s a small practical element. GRUB itself is quite a hefty tool to accommodate all kinds of boot setups, and it works well. If you have a simple boot setup though you could probably shave a couple of seconds off of the boot time just by using the simplified sd-boot and loading the kernel via its EFIStub.
A learning exercise in self-signing EFI binaries, enrolling a MOK, and setting up scripts to handle updates.
All boils down to my enjoyment of doing weird nerdy things though, ultimately. =)