You end up with a self-contained application binary which runs anywhere that you could run a Linux kernel.

However it's different from embedding an initrd into a kernel image in several ways: The kernel is minimal, containing only the bits which are needed by your application or drivers required to run on the hardware. The application runs in kernel space and is linked with the kernel which gives you a nice performance boost because you never context switch (which is the driver behind the whole project actually). You can also call kernel functions which aren't syscalls, eg. for specialized memory management.