The main feature Firecracker (and `crosvm`) is that they offer very trimmed-down VM's: you get disk, network, and serial (and KVM of course); that's it.

Thus if you don't need the emulation (and other extra features) from QEMU, it can be simpler to configure and certainly more secure to run.

For example I intend to run development VM's on Firecracker instead of QEMU.