How to configure SC dependencies in SLD (while creating DCs)

Hi

I have gone through some pdf but still i am not clear how and why DCs are used.... what is advantages of using DCs?

I am trying to work on DCs(Development Components)... i have gone through one PDFs... In that first they have created product and software component... and they define SC dependencies after that they define a track....

after finishing this configuration they have created the development components(DCs) in NWDS....

Is it neccessary to created product and software component and all... if its yes please tell me why?.....

I have created Products and Software components but i dont know how to create Dependencies and how to defice track....

So please give me a clear picture on DCs......

please help me

Thanks & Regards

Ravi Shankar B