A steam cleaner holds water in more than one place, sometimes a fresh tank and a separate dirty water tank, and both need to be fully drained or the unit can leak all over the box or the truck.
Pro tip: Run the unit briefly with just water after your last cleaning session to flush any cleaning solution residue out of the internal lines before draining and packing.
Watch out: Never pack a steam cleaner with water left in either tank; standing water can leak into the box, and in freezing weather it can expand and crack a plastic tank.