Or, What is the difference between blender ik rig and final ik?
Final IK is an IK solver that works in real-time.
An IK rig in Blender - or whatever DCC - is only used to aid in the animation process while working in the DCC, it has no impact once it’s imported into Unity at all.
Once you export a model with animations it bakes them down and the IK bones are all but worthless at that point. (That is why you should only export the deformation bones, in order to not clutter your model with unnecessary bones wasting performance)
You don’t “need” it, but it will improve your characters quite a bit by making it seem like they are actually touching the environment.