Why do lizards have sticky pads on the underside of its 4 feet?

They do not - and not many lizards can climb smooth surfaces. The pads on the lizards that can climb glass are covered in millions of tiny hairs/bristles and the ends of these have a tiny spot of liquid - like an oil. The phenomenon that is used to enable them to climb smooth surfaces is surface tension. The reason for the ability to climb smooth surfaces is that they are then able to exploit more habitat - like large shiny leaves.