Ballast, the oldest trick in the book
Trimming a vehicle by adding weight on purpose.
Ballast is deliberately added weight, carried for no reason except to correct where a vehicle's own centre of gravity or overall density has landed. It is the oldest trick in the book because it is also the crudest, a way of fixing a mismatch after it has happened, and that simplicity is why it has survived essentially unchanged from the earliest sailing ships to the newest submersible.
Absorbing the drift between drawing and hull
Every hull is built from real components whose weights and positions drift a little from the drawing: a motor sourced slightly heavier than specified, a cable run longer than planned, a housing wall thicker than intended. Each drift is negligible alone, but the sum rarely lands exactly on the intended trim, and trim errors of only a percent or two of total weight are large enough to see by eye once a hull sits in water. On a fifty-kilogram vehicle, one percent is half a kilogram, about the weight of a full half-litre bottle of water, and in the wrong place that is enough to leave it visibly bow-down. Ballast absorbs that accumulated error, adding mass at a chosen low position until the centre of gravity and overall density land where they were meant to. It is added in small, separately weighed increments, since overshooting in one lump just trades one visible trim error for its opposite.
Early sailing ships loaded stone or gravel into the bottom of the hull before a voyage for exactly this reason, and a ship that sailed without enough rolled dangerously. A hot air balloon pilot's sandbags do the same job in the opposite direction, mass carried only so it can be shed to nudge the balance between weight and lift, which is the same correction a submerged hull makes with its own fixed or adjustable ballast.
Room for weight nobody has weighed yet
Space for ballast, as low in the hull as practical, has to be designed in from the start, even though nobody knows how much will be needed until the finished vehicle is weighed and floated. A hull with nowhere sensible to bolt corrective weight ends up either mistrimmed or carrying its correction high or off to one side, reintroducing the stability problem the ballast was meant to fix.
Fixed ballast corrects only the condition it was set for, the vehicle as weighed on one day, and does nothing once heavier batteries or a new sensor go aboard. Many designs therefore carry some ballast as variable, pumped water or weights moved between mounting points, so trim can be corrected whenever the load changes. Every unit of ballast also displaces payload or battery, and a design that needs a large share of its own weight in corrective ballast is signalling that its mass and volume were never balanced properly in the first place.