Howdy
The notation in the Standard Catalog of Smith and Wesson by Supica and Nahas is 'apparently for the belt and suspenders kinda guy'. Really.
The explanation I have heard is that the idea was that when in close quarters with a bad guy, if he were to try to grab the top latch and open the gun to disable it, he would not be able to since the thumb latch must also be pushed forward to open the gun.
I dunno if I believe that explanation, but I have heard it.
These things were one of the last top breaks S&W designed. They were only produced from 1909 to 1920. The first hand ejectors with the now familiar thumb latch showed up in 1899. Notice the frame of the perfected model already has the modern shaped trigger guard integral with the frame. They always seemed to me to be kind of a transitional model between the top breaks and the hand ejectors.