I believe the WT originally introduced this for legal reasons. The WT has a history of having difficulty exactly how to define a member of the religion. It dates back to 1917 when Rutherford et.al. were on trial for discouraging the draft/selective service, which was illegal under the Espionage Act. They were issuing affidavits to brothers saying they were members of a pacifist faith, but didn't have any way to substantiate they were members. No record of baptism/consecration, no record of preaching, no ministerial training, nothing besides anecdotal statements that they attended meetings. Those affidavits were damning and were a big part of what landed Rutherford in jail. Before and during WW2 the WT had very little success in keeping the brothers out of jail over the draft issue.
Post WW2 they took a different tack, 'all JWs are ministers', introduced the pioneer status, hour requirements of preaching, baptism questions, baptism and preaching records kept, etc and they began to have comparatively more success in getting the brothers the 4-D classificaton. Part of the strategy was having a record that the brother was a JW and definitely not a member of another faith; hence the letter to officially break off from the previous religion.
My thoughts, anyway, others may disagree