It's worth noting, in addition to u/Jason-_B's excellent comment, that the placenta is not unique to mammals - it's seen in fish, lizards, and snakes as well. More importantly, unlike mammals, the intermediate states are still around, and plentifully represented.

In species with internal fertilization, the egg has to spend at least some time in the female regardless, just to add yolk and a shell. But more time in the female also lets her more precisely control the egg's environment, especially temperature, so keeping them interally has advantages (as well as the disadvantage of not being able to ditch them to escape a predator, and being "weighed down"). So a lot of species have variable time before laying, all the way up to laying right before hatching. Oxygen, CO2 and water can transfer, but it helps to ditch the shell in that case. However, no nutrient transfer occurs. At the very highest extreme, this is ovoviviparity - where the eggs entirely lack calcified shells, and the mom "lays" them immediately before or as the offspring are "hatching". From an outside perspective, this looks just like viviparity, but the key is the lack of nutrients - they need a yolk.

But if you've got eggs interally for a while, why not transfer some nutreints? There are lots of ways to do this, with the most bizarre probably being some species of caecilians (long, worm-like, burrowing amphibians) in which the mother grows nutritive lining in her uterus, which the young scrape from the walls and eat. However, a common way is to vascularize the yolk sac, squish it up against the uterus, let them fuse, and transfer nutrients across - bingo, you've got a placenta. Some of these are every bit as complex and specialized as mammal placentas.

The most useful thing is we have numerous independent evolutions of the placenta outside of mammals (who only evolved it once, as far as we know), as well as living examples of every intermediate you could ask for. There are even species (three-toed skinks) where some populations give live birth and others lay eggs.

Even crazier is the exception - Archosaurs (crocodiles, birds, dinosaurs, and their relatives) cannot ever evolve live birth. Unlike other species, the Archosaur embryo uses the calcium in the eggshell for bone calcification and, if the shell is removed, the hatchling is basically a gummy-bird or gummy-gator (obviously non-viable). This means they can never ditch the shell, and never take those first steps. And not a single Archosaur has ever evolved live birth, despite hundreds of millions of years of opportunities, and literally ruling the planet for most of that time.