Judging from the skeleton tree, these ik_foot, ik_hand bones are not connected to the pelvis.
My first questions is that why are these bones there? Why is the IK_hand_root bone is at the same location of the root?
The second question is, in some animations, when the character is moving, this ik_points that are not attached to the foot and hand bone actually moves according with this bones.
However in other animations of this exact same skeleton, these IK_points does not move at all?
Well for the most part the IK bones are legacy dating back to the original rig used in previous version of the unreal engines so the full reason is lost to history. Unreal 4-5 does have some nodes that do take advantage of the IK bones.
However.
The IK bones/joints have other names such in Motionbuilder makes use of whats called an Auxiliary joint so the IK bones can be used as a mans of interaction with objects when the need for two handed switching is needed as part of the main hierarchy. Les say you have a bolt action rifle you will need to constraint the weapon to the left hand and operate the bolt action with the right when it’s more common to have the weapon constrained to the right.
To have such a proper set up the IK joints are linked to their IK root at world origin and the IK roots are linked to the character root as part of he hierarchy so that they match Epic based animations with out errors as the IK joins needs to be included as part of the skin weights to prevent hierarchy errors.
Other uses included using the IK placements as targets as a means to adjust the arms using 2 bone IK to lock hand placements. Not so much as useful now that we have sockets that can be added once in Unreal but still useful when authoring animations if one wishes to key frame two hand use of say melee type weapons or as fixed link points. As to the IK joints floating that is usually caused when the IK bones are “not” constrained, not linked, to it’s matching parent and the animation baked as part of the export process.
Here is the result if you “constrained” the IK joints in MotionBuilder using them as targets as to connecting a weapon keeping both hands relative to their contact points.
How useful are they today?
Well lots of things become useful when authoring animations but needs context of what you are doing. Like juggling balls for example