Because it's "cleaner". Required partitions for UEFI boot: 1 FAT32 with boot files, 1 other partition with the OS. (some UEFI bios allow you to boot NTFS directly and you might not need the boot partition at all!) I actually don't know why MSR is a thing. AOMEI says it contains boot files, which is false. It's empty. But MSR might be used to store metadata in some disks configurations (dynamic disks). Never seen it in person. The recovery partition is optional. You can have your recovery files inside the main OS partition but if the partition is corrupted or encrypted, you wont be able to boot to a recovery OS without using an external boot disk (dvd, usb, network).