Part of the gondola building and maintenance process requires squerarioli to flip the boat over so they can get to the underside.
You'd think they would do this all above ground, but no, not the Venetians.
They dig a hole in the floor so the whole operation is lower to the ground.
Sure, you could set up planks, and do all your work from higher up, but for various reasons they prefer this method.
I'm just spitballing here, but do you think some old grouchy gondola builder fell off a plank centuries ago and made a new rule from that day forward? It could happen.
With a sprained ankle and a bruised ego, an angry guy with a shovel could dig a hole pretty quickly.