Why is there a dimple in the projectile?
The projectile in that cartridge is a typical “button-nosed wadcutter.” The cartridge is a factory match load, designed to cut very clean, edge-to-edge holes in paper targets to make scoring easier, and of course the whole design was one that proved to be very, very accurate. As to the design-purpose of the button nose, I really don’t have an answer. It is above my technical (or lack thereof) pay grade.
I always shot Winchester’s version when I used factory .38 wadcutters rather than my own handloads, so am not that familiar with Remington’s version, but I assume that bullet is copper-washed lead. (I don’t collect revolver cartridges).
This projectile and the plain lead ones were probably loaded for the Smith & Wesson Model 52 target autos. The 52 required a flush mounted projectile in order to fit in the magazine.