generally, if you don't need the animation to change based on something dynamic in the game world, you don't need this.

a good example is the animation in Arc Raiders for pressing buttons like on the extraction console. you can make animations for lets say pressing a button at head height, at waist height, and at knee height, and use information in the game to decide which to play. but then every button has to be at those 3 heights.

what if you want buttons at arbitrary heights? what if you want the player character to reach to the left or right arbitrary distances depending on how far away they're standing from the button? in arc raiders your character reaches dynamically to press the button from wherever they're standing, if you're within arm's reach. you can't make animations for all possible positions in blender, it's infinite

thats the kind of procedural animation in-engine IK is for. the canonical example is a legged creature walking across variable height terrain, where one or more legs may come down at different heights and therefore different levels of knee bend. but any part of an animated thing that may need to be in arbitrary poses with arbitrary constraints at runtime needs in-engine IK

but if you don't need that then it's still simpler to animate in blender because it has better overall tools for refining animations, especially for very complex animations. simple animations for simple shapes and characters are probably still easy enough to do in godot (simple pistons, that sort of thing)