It is a form of risk management that reduces the risk of losing all offspring through specializing too narrowly. Offspring that are most fit in the prevailing set of environmental conditions will have the greatest chance of surviving to reproductive maturity and thus contributing their genotype to the next generation.
For example, a plant that produces some seeds that germinate quickly and others that germinate slowly is hedging its bets against the possibility of drought. If there is a drought, the seeds that germinate quickly may die, but the seeds that germinate slowly may survive.
Similarly, a plant that produces some leaves that are broad and others that are narrow is hedging its bets against the possibility of high winds. If there are high winds, the broad leaves may be damaged, but the narrow leaves may be able to withstand the wind.
By producing a mix of different phenotypes, plants are able to increase their chances of survival in a changing environment.
