fal'Cie are godlike beings possess the power to curse humans, transforming them into 'l'Cie.
Each Fal'cie was created for a purpose (by one of the Fabula Nova Crysalis mythos's gods - usually either Pulse (Gran Pulse Fal'Cie) or Lindzei (Cocoon Fal'Cie), but note that Lindzei, Pulse, and Etro are themselves referred to as Fal'Cie created by Bhunivelze), and beyond the capacity to transform humans into l'Cie they each have precisely and only the capabilities they need to fulfil that purpose. In this respect, Fal'Cie are the ultimate expression of "form follows function".
One of Orphan's lines suggests that the function in question is the Fal'Cie version of a Focus - but unlike a human-made-l'Cie, they do not have consequences for defying or failing their Focus, but rather are literally incapable of doing so. A Fal'Cie simply cannot take any action that doesn't at least indirectly further its Focus, but its power to act towards that Focus is vast almost beyond comprehension. The problem comes when, like Orphan, "two irreconcilable Focuses [it bears]".
That, despite all the Fal'Cie's manipulation and coercion, humans and l'Cie are far freer than Fal'Cie are is a major underlying theme of the story.
Barthandelus's Focus was to rule Cocoon and orchestrate the attempt to open Etro's Gate - and to do that, he needed to be able to move around and appear human, so he was able to do those things, but he wasn't able to act directly to achieve the second, because it conflicted with the first - hence his need for Anima. Atomos's Focus was excavation as part of terraforming, with a suspected additional agenda of seeking the Gate beneath the earth, both of which again required mobility. Carbuncle's was food production, which doesn't need the ability to move, so all Carbuncle could do was float where it was and manage the hydroponic gardens. Likewise Kujata, with a Focus of generating power for use by Cocoon's populace, was immobile.
Orphan, by the nature of its mode of existence before the end of FFXIII, is a slightly special case, but the same principles apply.