As with spokes, there is a nipple to suit to every wheel requirement, you are viewing Black Alloy SILS Nipples.
The rounded design of the Patented Polyax nipple head reduces the stresses produced by axial and lateral forces (inherent in wheels with low spoke counts and crossings). Polyax nipples re-align themselves within a rim hole up to 9 degrees in a radial direction and 6 deg in an axial direction, making for a much superior fit.
Sapim makes nipples in nickel plated brass and in aluminium. All nipples are forged.
Arguments in favour of a brass nipple are price and rigidity. The strength/weight ratio is the principal argument for a 7075 alloy nipple. They are both superior to all other makes. Anodized coloured alloy nipples will help guard against corrosion as well as look great.
High profile or radially mounted wheels are prone to impacts which can cause momentary loss of spoke tension, allowing nipples to loosen. Sapim SILS Polyax Aluminum nipples (Sapim Integrated Locking System) have a special thread system which prevents loosening. A SILS nipple, which is self-locking, does not degrade over time, allowing wheels to be trued, re-tensioned, or re-dished with minimal hassle.