Natural secretions from the sebaceous and sweat glands can prevent the skin from exfoliating properly, causing a build-up of these dead cells. Minute particles of dust and pollution also collect on the skin as do bacteria. If the skin is not cleansed properly, breakouts or severe dryness could result. Skin care products and make-up must also be removed thoroughly as they can also inhibit exfoliation and encourage bacterial growth. This translates to a lot of make-up not being cleansed off.
Soap is the oldest cleanser, usually a mixture of sodium salts and various fatty acids.
Bar soaps vary in contents from brand to brand depending on the fats or oils used. So called neutral soaps actually are alkaline with a pH of up to 10 when dissolved in water. Soap is usually also found in shaving creams because the alkalinity helps to soften the beard for shaving. This is a measure of the Hydrogen ion concentration , and so the acidity or alkalinity of a solution. The skin produces sebum which forms a thin layer on the skin called the acid mantle.