On a standard keyboard the thumbs press one key between them and otherwise rest. Everything else falls on the little fingers, which reach for enter, backspace, shift and control despite being the weakest and least mobile.
A thumb cluster redistributes that. Three to six keys sit within a thumb's arc on each half, so the motions repeated most often during editing move to the digit best able to absorb them.
The cost is that thumb keys have no muscle memory to build on. Home-row letters transfer from your old keyboard because the fingers already know them; the thumb keys are genuinely new, and they are the most common reason people report still hitting the wrong key in week three.