My personal list has a center of 2 14-Man Tide Squads, 2 5-Man Intercessors w/ AutoRifles, Champion, Marshall w/Helm, Cenos and Deep Striking Castellan w/Teeth.
The existence of that centers defines my army play-style and style of engagement: melee anvil. My army is then built to support that style of engagement. However, why is the center important? How does it define your army? Well in a word, the center is your army.
That Gulliman list might have a couple units, perhaps a few backfield Devastators, or the generic Space Marine List a few forward deploying scouts. The Blood Angel list at LVO with its numerous Intercessors, was spearheaded by Hammer Captain, Mephiston and other Characters, alongside Sanguinary Guard and Death Company Bombs or Deathstars. Each Center presents is own unique challenges and advantages.
A classic or basic center of a double character (Marshal and Castellan or Champion for us) and 3-4 Line Squads is able to secure a position. It’s highly inflexible in movement, as you try to stay bubbled. That inflexibility renders is vulnerable to flanking. If an enemy say Scion Squad hits your left flank. A classic center needs to delegate or separate one of its line squad to handle the scions. A Razorback center instead would disembark its contents and send a couple MSU Squads to handle it.
But those MSU squads would risk the counter attack or the opponent’s BolterBro/(or equivalent fire) taking them out. Though, in theory, the amount needed is give or take take 50 Bolters Shot per 5 man squad, or 10 Shots a Marine. For context a 5 Man MSU with DoublePlasma puts out 25 Bolter Shots in Rapid/Charging.
In contrast a center like Fire Raptors, cannot take and hold objectives, reliant heavily on its supporting characters to clear enemy off midfield (at least those who cannot be shot off). But unlike the above examples given it has a larger array of weaponry can respond to flankers without separating from its support elements.
Where do as Black Templar fit into all this? Well, first and foremost, our Crusader Squads are one of the best power armored Line Squads in Warhammer 40,000. They can be whatever you want a line Squad to be. 5 Man MSU in Razors? 5-4 Rhino Fire Support? 13-15 maybe 20 Man, Tide Squads. The innate weakness of said Crusader Squads is battlefield flexibility. We lack combat squad and thus unable to respond to a changing battlefield scenario.
Tide Crusaders especially suffer that weakness. Meaning we have to build our army to incorporate and reduce that weakness. I use Mechanicus Skitarri; though Space Marine Biker Squads, Scouts or even cheap Assault Marine can fill the same function. In addition to my two tide Squads, I run two Intercessors Squads and Cenobytes. Intercessors armed with Auto Rifles, follow behind my Crusaders.
If the enemy force requires me to separate I move one of my two Intercessors to engage. And if I need to hold a midfield and keep moving on, I leave my Cenos behind to claim the objective. Third, the issue is getting to enemy force. Advancing turn 1, means I am likely within range (average distance is 7”-9” well within reason for our tactics). But I need to survive the critical first two turns.
So I utilize 10 reserve Reivers, Ponies, Scions, and Castellan w/ Teeth, alongside two Rattling Squads. A pure list could use Scout Marines, Assault Terminators, Interceptors, or more Reivers, to achieve a similar effect. But they are important as they force the enemy to spread out and being unable to bring their guns to bear on my Center army or force.
The flankers and deep strikers, work together to support my Center. If my Center were Rhino focused, I’d not have the flank support or as many deep strikers (Ponies notably). But I do it this way so the rest of list is setup to support them. If you have a weak center your list fails.
A lot of players I see take the minimum troops and then load up on toys. Minimum troop/squad size only good for Razorspam style lists. If you are taking a couple Storm Ravens or LRC, then throwing MSU Squads, maybe a single overly upgraded Vanguard and Ironclad Dread into the list, it’s gonna fail before it begins.
Furthermore different styled centers require different characters, Helbrecht and Grimaldus do not belong in the same list. Helbrecht supports high elite Pseudo Star Lists where they need his full re-rolls to bring maximum effect. Use Grimaldus instead for a black tide list for when his ability can become most relevant.
These character you take them or you have a list that can utilize them. In either scenario, your ‘army’, not a single unit defines your force. A good list starts with a firm foundation. In most lists, that should be the troop section.