One clause: friendly DEATH GUARD CHARACTER units have +3 inches to their Contagion Range, to a maximum of 12 inches.
To see the size of that, put it next to the base army rule. Contagion Range is 3 inches in battle round one, 6 in battle round two and 9 from battle round three. So this detachment takes your characters to 6, 9 and 12. In the first two battle rounds, when Death Guard are at their weakest, it doubles the radius on every character you own.
That is the argument for the detachment in a sentence. The army rule is a clock, and this is the only 1-DP way to move the clock forward.
Two restrictions to read carefully. It applies to DEATH GUARD CHARACTER units, so a Plague Marines squad with no character attached gets nothing, and a Character who dies takes their extended aura with them. And it is capped at 12 inches, which is the same ceiling Faction Pack v1.2 added to the army rule itself, so stacking this with Plaguesurge, Blooming Pestilence or the Tallyband Summoners rule hits a wall.
One correction, because it is repeated almost everywhere: the detachment rule does not grant a second Plague. That effect exists, but it is the Host of the Hybridised Pox enhancement, it costs 40 points, it is once per battle per army, and it only applies within Contagion Range of the model carrying it. Those are very different things when you are building a list.