Hi Malar,
I assume you mean "why" instead of "how"?
Because "how" is really depending on what do you need, there are tons of possible custom-developments, varying from tables, views, development_class, etc etc.
The "why" is because it's a naming-convention, so you don't change something that belongs to SAP (SAP Development), because then it's called a "modification" or a "repair", and the impact is very different than if you are creating in your own (Customer) namespace.
I hope this will answer your question.
Regards,
Andre