Because it always has a mechanical advantage greater then 1.
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The bottom of the bathtub is inclined. It leads towards the drain. They make it so that water will go straight to the drain and wont just settle. It would then take force to keep the water up the incline (away from the drain). That followes the definition of an incline plane.
The applied force will depend on the required force, and the angle to the ramp (or the horizontal) at which the force is applied.
Efficiency is the ratio of power output to power input. It is not the ratio of forces or distances. You can move a load using less force by reducing the gradient of the plane. But what you gain in reduced force you lose in having to travel a greater distance to raise the load through the same height. There is, therefore, no net advantage. The only way to increase the efficiency is to reduce the friction. This can be done by lubricating the plane, or load (or both), or by placing rollers between the load and the plane - so that there is rolling friction rather than sliding friction.