Have you ever poured an oil into water or vice versa? The oil just sits on the top of the water. As creams and lotions are used to deliver and hold nutrient rich plant oils, hydrosols, teas and other aqueous substances on the skin, an agent is need to blend or hold all of these ingredients together in a stable mixture rather than letting them separate. Hence the need for an emulsifier preferably a plant emsulsifier which occurs naturally as fatty acids in small quantities in plants and nuts. It is an ideal medium for blending fluids and other substances containing oil and water.
The emulsifiers are molecules with one hydrophilic "head" that dissolves in water and a hydrophobic "tail" that dissolves in oil. In this way the water and oil droplets are unable to separate out - the water encapsulates the oil molecules and the interfacial tension between the water and oil are neutralised so the molecules won't separate over time. In this way an emulsion is created ie a cream or lotion. In addition, plant emulsifiers improve the consistency and texture in the final product and act as softeners, emollients and moisturizers.
So in a nutshell, we need emulsifiers to keep oil and water from separating but with the added benefits of improving the feel and lubricity of the cream or lotion.