The correct response to the question is "To bring the firearm into compliance with ATF rules".
Any legal response should be accepted as there is no requirement codified in law as to the allowable reasons to build a SBR.
Plus, the ATF really doesn't have a "say" in the matter. Unless the reason is an "illegal" one, it should be approved.
It's just the ATF dragging their feet on approving applications...
To be honest, I was tempted to put "to finally make this thing legal!"
There's a rumor of a Form 1 on the Interwebz which has “zombies” as the reason, and it was approved.
Don’t know if it's for real, but it should be!!