Once the frames were constructed and glassed, they had to be leveled (unlike a house, leveled against the ground, the boat has to be leveled to the water line which gets tricky with a curved boat bottom and lots of physics involved). This is a critical process, like most of these initial build-steps. If it’s not done correctly the first time, the consequences are expensive; corrections potentially compromise the integrity of the boat.
When the frames are leveled, their stands (I think it’s called the strong-back) need to be braced. In this construction, we’re cold-moulding all the wood around the frames using clamps (a crap-load of clamps!), so the frames MUST be solid so the hull of the boat does not warp and bend under the stress of wrapping wood around it.