I think it makes sense to me though:
1) unmount the datastore if you want to get rid of the LUN (followed by deleting it on the array side)
2) delete the datastore if you want to get rid of its content, but want to reuse the same LUN by recreating a new datastore on it.
My hands-on experience is too limited to comment on the likelihood of going into APD in scenario 2 (followed by removing the LUN), but arguably scenario 1 just works.