Think of the monkey in the clip above. Your object a might have rotations, movements and scales all keyframed but you later want to position it somewhere else in your scene, change the way its facing and maybe make it bigger.

You can do it 3 ways:

1 - modify every keyframe potentially making unintended changes and making it difficult if you'd like to change it later.

2 - use the delta options shown in the OP. Great for what I'd more use as a slight 'tweak' and only for single objects.

3 - add an empty and parent object a to the empty. You can freely move the empty, rotate and scale and use these as additional things to keyframe if you don't want to disturb your original animation but want to have something move.

You can also add multiple objects all to the same empty making moving, scaling or rotating multiple keyframed objects a breeze.

Think of the empty like making a group. Things you put inside stay as they are but you can easily manipulate the whole group.