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Pensions, annuities, and “retirement”
An annuity is a product. In simple terms, you hand over a lump sum of cash and receive an agreed annual income until you die. The underlying investment required to reach that income level is not your concern, it's the provider's worry. So there is a huge mount of security to the retiree in having an annuity. It is worth pointing out that with simple annuities where one gives a lump sum of money to (typically) an insurance company, the annuity payments cease upon the death of the annuitant. If any part of the lump sum is still left, that money belongs to the company, not to the heirs of the deceased. Fancier versions of annuities cover the spouse of the annuitant as well (joint and survivor annuity) or guarantee a certain number of payments (e.g. 10-year certain) regardless of when the annuitant dies (payments for the remaining certain term go to the residual beneficiary) etc. How much of an annuity payment the company offers for a fixed lump sum of £X depends on what type of annuity is chosen; usually simple annuities give the maximum bang for the buck. Also, different companies may offer slightly different rates. So, why should one choose to buy an annuity instead of keeping the  lump sum in a bank or in fixed deposits (CDs in US parlance), or  invested in the stock market or the bond market, etc., and making periodic withdrawals from these assets at a "safe rate of withdrawal"? Safe rates of withdrawal are often touted as 4% per annum in the US,  though there are newer studies saying that a smaller rate should be used.  Well, safe rates of withdrawal are designed to ensure that the retiree does not use up all the money and is left destitute just when medical bills and other costs are likely to be peaking.  Indeed, if all the money were kept in a sock at home (no growth at all), a 4% per annum withdrawal rate will last the retiree for 25 years. With some growth of the lump sum in an investment, somewhat larger withdrawals might be taken in good years, but that 4% is needed even when the investments have declined in value because of economic conditions beyond one's control. So, there are good things and bad things that can happen if one chooses to not buy an annuity. On the other hand, with an annuity, the payments will continue till death and so the retiree feels safer, as Chris  mentioned.  There is also the serenity in not having to worry how the investments are doing; that's the company's business. A down side, of course, is that the payments are fixed and if inflation is raging, the retiree still gets the same amount. If extra cash is needed one year for unavoidable expenses, the annuity will not provide it, whereas the lump sum (whether kept in a sock or invested) can be drawn on for the extra expense. Another down side is that any money remaining is gone, with nothing left for the heirs. On the plus side, the annuity payments are usually larger than  those that the retiree will get via the safe rate of withdrawal method from the lump sum. This is because the insurance company is applying the laws of large numbers: many annuitants will not survive past their life expectancy, and their leftover monies are pure profit to the insurance company, often more than enough (when invested properly by the company) to pay those old codgers who continue to live past their life expectancy. Personally, I wouldn't want to buy an annuity with all my money, but getting an annuity with part of the money is worthwhile. Important: The annuity discussed in this answer is what is sometimes called a single-premium or an immediate annuity.  It is purchased at the time of retirement with a single (large) lump sum payment. This is not the kind of annuity that is described in JAGAnalyst's answer which requires payment of (much smaller) premiums over many years. Search this forum for variable annuity to learn about these types of annuities.