Cognitive planning
To be able to pick up an object, first, your baby must plan what they are going to pick up. Then, once in their hands, they must plan where it is going to go. This is a skill that will build in time – you might notice that your baby simply drops objects at times, this could be because they have already seen and planned what they want to pick up next.
Sensory information
When picking up an object, your baby processes sensory information about the feeling of the object in their fingers. They must navigate the size and shape of the object in relation to their fingers.[1]
When placing items back into containers, there is the additional sensory challenge of fitting the object in. Your baby might have to manipulate the object for it to fit into different sized and shaped containers.
Tactile perception
Your baby experiments with many ways of moving their fingers in order to successfully pick up and place objects back again. They are likely to pick up a wide range of things, made of assorted materials, so their fingers will need to determine how much grip to apply to each thing.
When first exploring how to pick up and remove objects from a container, there will be a lot of trial and error.
Managing balance
Whether your baby is sitting, lying or standing to take objects in and out of containers, they will need a degree of balance. In most instances, their upper body will move, leaving the lower body still. This can cause them to topple over. You'll be able to see now just how hard their muscles are working to keep their body upright and balanced.
Reference:
[1] S.J. Edwards, D.B. Gallen, J.D McCoy-Powlen, M.A. Suarez (2018). Hand Grasps and Manipulation Skills. Slack Incorporated