This vest is part of a matching set for a preemie.
Preemies often cannot maintain their body temperature, so layering is necessary, even in summer heat. Also, while in the NICU, and even at home, they often have tubes or catheters or monitors that need to be adjusted and having clothing without closures helps with that.
This is a nice, simple garment to make in one piece. It has enough interest to keep a worried mama or papa occupied, but is not so complicated that it requires too much attention.