When you replace the operating system kernel, it is far easier to reboot the operating system than to try to replace the kernel while running.
It is possible to replace the kernel while the system is running, but the consequences would mean either internal-kernel data structures cannot be re-organized or routines must be written to modify data structures in-place before running new code.
Replacing the system standard C library might justify the reboot: you have to restart all processes on the system to take advantage of the new C library anyway. But, you can just
/sbin/telinit u to force init to use the new library, restart all your services, then restart X if you're running it, and be done, without a reboot.
So, it's just the kernel that requires it.