Why do water pipes have a large bend in them?

Typically, it is drain pipes that have bends, variously called J-traps, P-traps, U-traps, or S-bends for their shape. They are designed to prevent drain line smells and sewer gases from returning into the home or business - the bend retains a water level above the lowest point of the bend on both sides. They also provide a slim chance of preventing valuables such as rings from being carried away.