Hellfire Cannons in the Lost and the Damned list are a strange beast – originally stemming from the Cannons of Khorne daemon engines they are now an Undivided artillery piece. At 200pts for a formation of four they are quite cheap and, were it not for the 1:1 Coven to Support ration, would be quite easily spammable. They come with a fairly resilient 4+ armour save and an Invulnerable save, meaning they can take a few hits without worrying too much, while their 15cm move allows them to garrison – something I continually forget to do!
The main weapon is a 75cm range, 4+ Macro-weapon, which makes it a fairly long range and hard hitting unit. Both of these things (ranges over 30cm and Macro-weapon) are in pretty short supply in the Lost and the Damned list, so the Hellfires fill a gap in the list well. Given there are four stands in the formation you are looking at 2 hits, and probably 2 kills, meaning three blast markers on any unit within 75cm which is pretty good. They also have a 4+ FF value which is decent.
I like to garrison the Hellfires quite far forward to make the best use of their range, while trying to get them placed in area terrain for the -1 to hit. It is unlikely they will move during the game if they can cover a fire-lane so I am less worried about dangerous terrain tests (Hellfires are armoured vehicles). Having a formation or two spread across your front line, near objectives, and in cover can help to stall any of your opponents armoured formations – especially things like Predators or Eldar Grav-tanks that lack Reinforced Armour. Hellfires can also help the Lost and the Damned with the smaller, speedier, titans like Warhounds and Revenants – putting a couple of hits / downed shields / points of damage on them.
In a list taking Hellfires is hampered by the 1:1 ration mentioned above – if I am having to spend 200+ points on a Stigmatus Coven to unlock a support choice, then I tend to want to take a hefty support choice like an Armoured Company or Artillery Battery. I tend to either take the Hellfires in the place of an Artillery Battery (though I do think the Basilisks, although more expensive, are better), or because points are tight. At only 200pts, they represent quite a threat to lightly armoured vehicles, and do it at a range that is not common to the list. Once I had one Artillery Battery in a list, the choice between Hellfires and a second Battery would be a close one.