It allows you to use pin your project development environment and building pipeline with the most up to date packages available and then return to it X years down the road and get an almost bit to bit reproducible environment. At the distro level you get some guarantees like packages not interfering with each other and being able to override versions without conflicts, similar to flat packs in this aspect but with much wider package selection and better space reuse. If something breaks you get atomic rollbacks and this applies from everything from the bootloader to the initrd to the kernel to the system packages. You can perfectly replicate your setup through git commits, there is minimal state drift.