Consider twins. My understanding is that a parent may only have one infant in their arms, the other infant needs a seat.

Nevertheless, a parent may choose to book a seat for their infant to give themselves extra space. If the airline puts that seat in a different row, it defeats the purpose.

They may also book a seat so they can use a carseat, which they may be traveling with anwyay, and also because it's safer for the kid to be belted in, and most small kids are used to them and they will fall asleep in them.