Is BofA going to redline all of their staff? No. But absorbing a spike at a single department or branch is another story, and it wouldn't surprise me if having excess capacity already on hand is more efficient overall than trying to reassign individual workers on demand.
Besides, actually working on a team that's redlined is exhausting, and doing it for more than a few weeks is a recipe for burnout. There's real business value in high employee morale and a low turnover rate, and having some amount of built in slack at normal operating capacity seems like a relatively cheap and reliable mechanism to safeguard that value long term.