If a player in 40k decides to build their army in such a way, they can unlock special rules for their units that will make them greater than the sum of their parts. These special lists are called Armies of Renown and they're coming in the War Zone Charadon book. Have a look for yourself.
From the article:
While many of the forces fighting in the war-torn Charadon sector are fairly normal* – the Adepta Sororitas are mostly still setting fire to everything, for example – others are notable for the less orthodox approach they take to war. Now you can adopt their unique tactics with a trio of codex supplements – for Freeblade Knights, the Drukhari Wych Cult of Strife, and forge world Metalica – and the never-before-seen Armies of Renown system.
By choosing to build an Army of Renown, you’ll unlock powerful additional rules for your units in return for a few limitations on what you can bring to the table. We’ve picked a couple of our favourites from the feature-stuffed War Zone Charadon Act 1 to show you what kind of things you can expect from this new system.