why do we need to learn about atomic structures ??????????????

Atomic structure is basically the foundation of chemistry. 

It is very important in order to understand :-

(a) The stability of atom

(b) why behavior of one element is different from other elements in terms of both physical and chemical properties

(c) why they react or combine with other elements to form molecules.

If we don't understand the basic then we won't be able to account for the diverse properties of different compounds, how they are formed and how they behave.