The goal is to move away from initrd images being generated on the installed machine where possible. The initrd is generated while building the kernel package instead, then shipped as part of a unified kernel image (UKI).
A unified kernel image is an all-in-one efi binary containing kernel, initrd, cmdline and signature. The secure boot signature covers everything, specifically the initrd is included which is not the case when the initrd gets loaded as separate file from /boot.
Main motivation for this move is to make the distro more robust and more secure.
Supporting unified kernels for all use cases quickly is not realistic though. Too many features are depending on the current workflow with a host-specific initrd (and host-specific kernel command line), which is fundamentally incompatible with unified kernels where everybody will have the same initrd and command line. Thats why there is 'Phase 1' in title, so we can have more Phases in future releases.