bc of his theory we know a baby only has the id developed and so when it cries it need something, also we know that the baby is at the stage where it has no concept of time, empathy etc so its not conciderate of others and its not able to wait until certain times etc
from his theory we know that soon the ego develops which is the stage where the child becomes less ego centric and is able to show sympathy and concideration for others, and is developing understanding of situations
by 5 year old the superego is developed this is the stage where children are able to make moral judgements from previous experiance-the child may want a cake but asks for it instead of steeling it as they have been taught steeling is wrong, here they may be becoming more awear of expectations, their religion and may also be judgemental of others who arnt the same (unless taught to be accepting)
by knowing these stages, by knowing that if the id ego and superego arnt balanced that the child may grow up to have problems and by knowing how we effect the childs personality, we are able to provide care and play experiances etc that meet the childs individual needs etc we are also then able to ensure we influence the childs personality in a positive way.
heres a good site to quote from:
hope this helps