One of the big problems of reproduction is that your offspring tends to be small and vulnerable. The longer you carry it in you, and the bigger it is when it separates, the more vulnerable you become. Animals solve this in a variety of ways: some just produce a whole lot of offspring, others have few offspring and guard them. Eggs are a partial solution to this problem: separate the offspring early and at least you don't have to carry it with you - but the problem of protecting them remains. As you probably know, eggs are delicious.
Let's say there's some (alien) creatures that are fairly similar to birds: they are active hunters, and they pair-bond for a season. Their problem is that they can't carry their offspring with them for as long as they'd like, as that'd make them too heavy. But if they lay eggs, one of them has to remain behind to keep them warm and defend them until they hatch.
Instead, the female births a single creature, a variant female. This creature is sessile and not very bright, but very much able to defend herself. In bird terms, think of a half-blind bird with atrophied wings and a very nasty beak. Genetically speaking, she is a clone of the female. The male then inseminates this variant. Both male and female then are able to find food for the variant, who sits in the nest, with the offspring maturing inside her. Anything that comes close to her and isn't bearing food is pecked to death with extreme enthusiasm.
After a fairly lengthy gestation period, the variant gives birth to the offspring, dying in the process. The offspring feed on her body, and are then pretty much ready to support themselves independently.
Sound far-fetched? Not really. There's plenty of animals where the mother dies after giving birth, and plenty of animals that have vastly different physical forms depending on all kinds of factors. Nor is it unheard of that one animal produces a second which then gives birth to a third.