A preservative is a naturally occuring or synthetically produced substance that is added to a product such as foods, pharmaceuticals, paint, biological samples, wood, etc. to prevent decomposition by microbial growth or by undesirable chemical changes.
Anything that contains water (i.e. cream, gel, lotions, etc.) is a paradise for bacteria, yeast, fungi and moulds and the natural sugars in plant extracts are their favourite snack foods. Bacteria is a fact of life in cosmetics, foods and pretty much any organism that contains even a trace amount of water.
Cosmetic products become easily contaminated by bacteria and fungi. The container (the product is in), oils, peptides and carbohydrates that make up cosmetics are a very good medium for the growth of microbes. All these factors contribute to the fact cosmetic products need preservation to prevent microbial growth and the spoiling of the cosmetic product and also to eliminate infection to the skin.
Now whether you choose a natural or synthetic preservative…Well that is one HUGE can of worms that I will not open as the debate on that subject will last forever… All I can say is that do your research and choose what you feel will best suit your needs!