About command Seals and Hougu

[Sire]

Midgame (chapter 3) spoilers:

What up with masters using command seals when ordering their servants to use their Noble Phantasms?

At first I thought Iori had used his seal to make saber never user hers and this is why
he uses another when allowing for the use. But then I remember that Lancer's master also used one when making her user it at the end of chap 1, and then Zheng also uses one for archer.

Is there any reason for the use of the command seal? Is it explained later or whatnot?

Or was the use of the command seal simpy to indicate the Noble Phantasm would be enhanced by the mana of the seal?


Extra question: How do rogue servants come to our aid? Are they simply travelling though leylines to our destination? Usually teleportation of a servant would require a command seal but we don't have one for them. Is it simply a "they are waiting for us to call and then when we do they come to our position on their spiritual form" situation?

[Samurai Boy]

My toughts is that it is like FGO where you use a seal to immediatly get enough mana to deploy a NP since it is rather cost heavy or instead it is to enhace the damage done by it.

[pokedude900]

Command Spells are essentially supercharged mana batteries. Normally if a Servant uses their Noble Phantasm, they'll be spent for at least the rest of the day unless it's a particularly weak one. Aside from that, Command Spells can also make the NP more powerful.

And I think the Rogue Servant support works similarly to the in-universe explanation for the Servants deployed in battle in FGO. They're not the actual Servants themselves, but shadow copies of them that are simply lending their power temporarily.