The article is written by a physical therapist, so it mostly focuses on motor and sensory concerns, but I'll also add that moving one's body freely and being able to explore things is necessary for development of language, cognitive, self-regulation, and social skills. Early developmental skills like head control, trunk control, and a sense of where one's body is in space can help with things later in life, like paying attention, following through with tasks, and understanding limits. These things also help with overall maturity; a child has to be aware of him- or herself before the child can be aware of others.
Oh, and this wasn't mentioned, but babies from newborn onward need access to toys they can manipulate with their hands. It doesn't have to be anything fancy. These can even be provided to stimulate development when the baby is in a container!