What problem does this feature solve?

This helps in adding dynamic watchers for dynamic component data

What does the proposed API look like?

watch (
()=> dataObject['propA'],
(prop, newValur, oldValue) =>{},
options
)

try this?

watch(
  toRef(dataObject, 'propA'),
  ...
)

There isn't always a name property to be passed but you can do that by using an array:

const name = ref('dynamic name')
const object = reactive({})

watch(() => [name.value, object[name.value]], ([newName, newValue]) => {})

try this?

watch(
  toRef(dataObject, 'propA'),
  ...
)

propA is dynamic. Unknown until runtime.