The idea of fairness creams has emerged from the worldwide preference (mainly in South Asia) for fair skin. White skin has a power of superiority and even it influences on the marketability to marriage partners, especially for girls.
Initially the manufacturing of fairness creams started internationally, these days Indian and South Asian companies are playing an important role in the cosmetic markets. In the cosmetic industry only fairness creams have been accounted approximately 40% of the total profit.
The science behind fairness creams is more interesting and being exploited by different companies in different ways.
Science: Melanin pigments are responsible for your skin colour. More dark means having more melanin pigments in your skin. Though they cause dark skin colour but they prevent your skin and eyes from harmful UV-B which you will get exposed whenever you are going out. Melanocytes are the cells which synthesize melanin and then it transfer melanin to the keratinocytes which are present in outer most layer of the skin. In the skin, transfer of melanin occurs after exposure to UV radiation, causing the skin to visibly tan and this pigment can dissipate over 99.9% of absorbed UV radiation. For the same reason why very fair or albino people get melanoma (skin cancer) easily.
So not only synthesis, the transfer of melanin from melanocytes to keratinocytes is also equally important. Whenever you are getting exposed to the sun, i.e. UV-B, another cell type called fibroblast will release a factor which then provokes melanocytes to transfer more melanin to the keratinocytes. The primary and acute sign of sun-exposed is the redness of the skin due to dilated blood vessels and overexposure leads to tanning. So, more melanin is the way of protection from harmful UV-B which equals to darker skin.