It solves many problems for operating system and bootloader developers. BIOS is old and quirky, it requires the system to boot in 32 bit legacy mode and then switch to protected and then to 64 bit mode. It requires running an x86 CPU emulator to run the vesa bios code while the os is running. Booting from a CD requires a bootloader that fits on a floppy image stored in the CD and the BIOS will emulate a floppy drive. Typically bootloaders will not need a partition, but just use some hard disk sectors near tho start of the disk and hope you don’t decide to create a partition there and overwrite it.
You don’t get to see all of this as a user, because operating systems worked quite hard to integrate it in a way that makes sense (most of the time). But it’s not uncommon to see boot problems because a partition wasn’t activated, the mbr bootcode was not installed, etc.
UEFI does solve very real problems and makes things simpler for us. Now we have some work to do to forward this simplicity to end-users