The apocalypses formation Purge Squadron for sisters of battle made you take 3-6 Immolators with 3-6 Retributor squad. The only real restriction was that the Retributors need 4 heavy weapons and 3 of them have to be heavy flamer,multi-melta, and heavy bolter. What you get for this formation is relentless and 6 or more models can combine their shooting attack for the use of Wall of Fire and if any of the squads passed their Act of Faith for rending they all counted as having rending for Wall of Fire.
Now that Immolators have a firing point out of the top hatch, if you only took the minimum for the formation of 3 Immolators with 3 Retributors you would need to have the squad get out. Now they can combine their fire from inside, it might not be the biggest buff but it is a tiny bit of improvement on the survival of the squads because they now need to be knocked out of their transports to be killed.
This most likely won't come up in a lot of game since the FAQ also said that the apocalypse formations are strictly apocalypse and can't be used in normal games but it is a nice little change if you ever do get to use it.