The "pinned" refers to a cross pin driven through the barrel and frame, located at the front of the topstrap. "Recessed" refers to the cylinder chambers, which are counterbored to enclose the rim of the cartridges.
Value? Practically none with modern ammunition. The recessed head of the case was to contain the gasses in case of a ruptured case head. And the pin prevented the barrel from rotating in the frame.
The pinning is unnecessary if the barrel is the original factory installation, as the fit is generally a crush fit. However, it the barrel has been removed, it is best to have a gunsmith pin it. Barrel can, and do, turn in teh frame, especially with heavy loads.
Don't know if you can see it or not, but top gun is pinned, look closely. Also cylinder is slightly longer, cartridge rimes would not show if loaded.