Why?
I have carried out numerous DSD's, both pool only sessions and also pool and OW session.
For some people, they decide that diving is not for them, for the majority, it is the final confirmation that they want to be certified.
I'm not speaking to Thal here.....
As outlined by the OP with a 6:1 ratio neither would I.
Fundamentally it is a "trust me dive". However with the right site, conditions, depth under the control of a dedicated scuba professional it is far removed from the essence of the discouraged "trust me dives". I feel it is a valuable tool for determining aptitude of OW candidates.
Pete