The purpose of the sleeping pad (or sleeping mat) is to insulate the sleeping person from the grounds coldness and humidity.
Depending on the thickness and the quality of the sleeping pad it also makes it more comfortable to sleep than on the hard ground.
There are two kind of sleeping pads: the first type is made of cell foam and the other type is inflating mats. The inflating sleeping pads can be manually or self inflating. Cell foam sleeping pads are naturally cheaper and also more reliable because of the solid indestructible build. The insulation of the cell foam pads is a bit better than in inflating mats.
Some more expensive inflating sleeping pads can include down which increases the inflating capabilities and sleeping comfort. The risk with the inflating sleeping pads is that the surface can be damaged easily, that’s why you should always have a patch kit with you to repair the possible damages.