Triarch Praetorians are the equivalent of Assault Marines, Necrons with anti-grav jump packs. They're also pretty valuable as agents of the ruling body of all Necrons, the Triarch. In this way, they are like a fusion of Imperial inquisitors and the Adeptus Custodes in their duties.
Why it Sucks to Be Them
Following the brouhaha that is the War in Heaven, the Necrons decided to go to sleep for fuck-all many years. During that time these chumps, becoming the emo that is Necrons now, decided "Fuck sleep! Let's go guard shit as penance!" and kept a VERY long watch on the hibernating 'crons and the protection of their culture. Apparently, they were good enough that even spess elves couldn't screw them up, despite literally millions of years spent trying.
During this time they taught some of the younger races the ways of the Necrons, as they figured keeping their culture alive is good enough to fulfill the Silent King's final order.
However, with their brethren now awakening, they decided to slam the alarm on as many Tomb Worlds as they can, making sure every lazy bum gets their beauty rest ruined. As the special agents of the ruling council, they uphold the rule of the Triarch, and see to it that the traditions of the Necrons are followed, disposing of any who challenge them with a single blast of a Rod of Covenant or a Particle Caster. This means they are effectively Alien Egyptian Zombie Robot Commissars. I believe I do not need to explain why or how this is awesome.
Because dropping from orbit is against Necron policy, and Praetorians don't have fancy teleportation built into their chassis, they can't deep strike in 8E. While on the ground they're noticeably slower than most Jump Infantry, but at least they're made from Lychguard bodies so they can take a beating. Unfortunately, they don't belong to any Dynasty, so they can't receive any Dynastic buffs, which are most of the buffs available to the Necrons.