She will produce more milk, maintaining the infant's health. And she will be better able to carry and defend her infant if she is healthy and strong, which generally result from eating well. The most important variable in most male primates' reproductive success is how often they mate. A male can father very large numbers of offspring if he mates often. But a male could also father no infants if he fails to mate.
Receptive females are always scarce, while active males are plentiful.
So, while females can easily find a mate when necessary, males have a lot of competition and may not succeed in fathering many infants. So anything that helps a male to mate more often will be favored by natural selection. Females only need to mate a limited number of times in their lives, since most of their adult lives they are either pregnant, caring for an infant, or rebuilding strength for the next one. Since it benefits a male's reproductive success to mate as often as possible, males are always ready to mate. So receptive females are in relatively short supply, and can always find a male when necessary.