An Arcanist workflow downstream requests information on leases visible to the user. The user is only interested in leases for which they are the owner, so Arcanist iterates over the results and returns only the ones with a matching ownerPHID. It's so much cheaper to have the server do this filtering than the client.
The proposed change will be small but necessary for the above feature, and will allow conduit calls to drydock.lease.search to specify an ownerPHIDs array as a constraint. Per the use case above, the workflow will provide the authorized user's PHID as owner, but I'm sure this change opens the door for a lot of people's wishlist features.