A one piece with proper underwire construction changes how the suit sits on your body. The wire provides lift and separation instead of the fabric just trying to handle it on its own. This is not a detail. This is the difference between a suit that works and one that requires constant adjustment. Underwire in a one piece means you get actual support at a reasonable price point. The wire needs to be sewn correctly and the fabric around it needs to have enough stretch to accommodate the structure without riding up. Look for suits that address the armhole situation too. A good armhole plus underwire equals a suit that actually stays in place. These are the one pieces that work because the engineering is correct.