The Ordos Necros is only five Inquisitors strong and shrouded in mystery. What's more it has an Ordos of unknown size specifically to watch over it. There are a number of theories as to the intent and area of study, here are just a few.
The most straight forward theory is the Ordos Necros studies death, simple as that.
The most fluffy theory is that the Ordos Necros are death cult assassins of so much skill and loyalty they were inducted into the ranks of the Inquisition and have hence forth dedicated themselves to bringing the art of the sword to as many planets as possible and possibly designing and researching new methods of blade based combat. It also makes sense that other Inquisitors would want to keep an eye on them.
Based on the hazy link between 'Necros' and 'Necron' some believe the Ordos Necros study the Necrons, which would explain why they're so secret as so far no technology has ever been stolen from the Necrons, aside from phase weapons.
- However, this connection can also be drawn from the Deathwatch (RPG), where there exists a faction within the Ordos Xenos called the Dead (ie: Necro) Cabal which consists of only Five members. Their purpose was not explicitly to deal with Necrons, but a mysterious Dark Pattern of events which are prophesied to culminate in the "End... (ie: Necro) ...of All Things". Though they were initially passed off as potentially dangerous eccentrics, it makes sense that other Inquisitors would be monitoring them carefully when things start going as predicted.
- The awakening of the Necrons was just one event that they predicted in advance as the prophesies date all the way back to M33. They may also have some connection to the missing Ordo Chronos, as two of the members have no history at all, one of them identified by a crystal hourglass and both just seem to have "appeared" when necessary knowing exactly what/when/where is about to happen. So, basically, the Imperium literally cannot lose since those two members will know what didn't work "last time" and the Imperium can just keep trying different things until it works against anything that would have otherwise destroyed it.
Another theory is that they study the slow death of the Emperor, and find the ways to stop or at lest delay it, as the Golden Throne falls apart and cease to function. From this point of view making entire Ordos to oversee the guys tasked with saving Emprah himself is a perfectly sensible move.
Finally the least likely but perhaps most interesting theory is that the Ordos Necros studies the Legion of the Damned, each as secretive as the other, it would make sense for the Inquisitors to want to keep track of the comings and goings of these mysterious zombie/ghost space marines.