Overlapping doesn’t happen (initial tweens are cancelled by the newest) and it allows Roblox’s native C code to handle the interpolation for us (linearly without slerp). The performance is no different than setting the CFrame each frame ourselves. However, a tween provides a smoother movement than raw setting or using a manual lerp. With a tween, we gain adjusted speed of interpolation before it gets overridden by the next.
See for yourself and try setting the CFrame on its own. The difference in experience is very noticeable.