The sear deactivation lever was specifically designed to allow owners/users to be able to release the striker without having to pull the trigger. This isn't an inconsequential feature to be able to provide to potential LE users, so the pistol line that eventually received the resurrected name of "Military & Police" had it incorporated. Obviously, S&W doesn't recommend releasing the striker by pulling the trigger, but by using the sear deactivation lever.
Now, as to having to make sure the lever is lowered in order to install the slide & barrel? If I had to guess, I'd lean toward it being because of how the lever was originally designed, and how the manual was probably originally written.
The original lever had a dog-leg angle and was "shorter". The angle made it easier to move with a tool or pin punch to release the striker for field-stripping, but it also introduced an unexpected potential problem. If it wasn't left in the down position, it could get in the way during reassembly (the slide's pick-up rail would catch against it).
By straightening the lever, it cleared the slide's pick-up rail during reassembly if it was inadvertently put back in the raised position. (It slips beside the pick-up rail.) Yes, it did make it a bit less easy to initially catch and depress the straight lever (without practice).
This picture shows an old (original) style sear deactivation lever on the left, and the new (current) style on the right.
Basically, after the pistol was released they discovered they needed to make the change to that lever to help make reassembly after field-stripping more "cop proof".
