Lifecycle funds were first introduced in the 1990s (Fidelity introduced their first target date fund in 1996), but their popularity has increased significantly in recent years.
These funds are aimed at people planning for retirement and have appeal because they provide a guaranteed value at a specific time. For instance, a 2020 target dated fund provides exposure to some form of growth security (US equities, international equities) while guaranteeing the original investment provided the fund is held to maturity. In some cases, these funds also have a guarantee that rises with the market value of the fund, locking in gains monthly, or in some cases daily.
For investors who will be highly dependent on their retirement savings (rather than a company pension plan, for example), the security of the principal guarantee is appealing.
Because the issuing organization is contractually obliged to meet the guarantee commitment, they alter the funds' allocation to emphasize cash and fixed income elements as the funds approach their maturity dates. By reducing exposure to the growth elements, the risk of market value being lower than the guaranteed value at maturity diminishes.
The funds are not without their critics, who point to the tendency for additional costs to be associated with target date funds, the reduced opporunities for growth over time, and the fact that the probability of negative returns from equity markets in general over a period of several years is relatively low.