Lots of good reasons to have the capability. To mention just a few.
Flexibiity to have more options when the larger 330MRTT is neither suitable nor available.
Suitable when only a small offload is required in support of operations such as QRA.
Suitable for tactical remote ops, such as in Africa, where no large airfields are available and transit times make MOB support impractical.
Suitable to support helicopter AAR which is impossible in the Voyager/MRTT flight envelope.