What does Sarty think when he says the house of major despain?
Concerning Faulkner's "Barn Burning," I assume you mean "sees" rather than "says." I'll answer accordingly.
Sarty's reaction contributes to the work as a whole in several ways. It provides unity when Sarty compares the house to a courthouse, referring backward and forward to the courthouse scenes. It also reveals where Sarty's mind is, figuratively: on the courthouse. His reaction also emphasizes the hopelessness and poignancy (a quality of specialness) of and in Sarty's position: essentially, his reaction is mostly one of relief--this house, these people, are too big to be hurt by his father, Sarty thinks. He is only a wasp to them. The house is beyond Abner, Sarty thinks. He is relieved:
...They are safe from him. People whose lives are a part of this peace and dignity are beyond his touch, he no more to them than a buzzing wasp: capable of stinging for a little moment but that's all; the spell of this peace and dignity rendering even the barns and stable and cribs which belong to it impervious to the puny flames he might contrive...this, the peace and joy, ebbing for an instant as he looked again at the stiff black back, the stiff and implacable limp of the figure which was not dwarfed by the house, for the reason that it had never looked big anywhere and which now, against the serene columned backdrop, had more than ever that imperious quality of something cut ruthlessly from tin,...
Sarty's reaction also, then, renews the image of Abner as metal, as tin, without heat (visible anger), but hard like tin.
Sarty's reaction also provides foreshadowing, of course, since Sarty will end up being wrong--not even this house and family and barn are beyond Abner's reach to maintain his dignity in his warped way.