Why should we bother about Extensive properties in Thermodynamic systems? and Why do we need specific value of some thermodynamic property? and what does it tell us about the system? Eg. specific heat capacity? How density of a thermodynamic system is a intensive property?
It's hard to answer your questions because they are not very specific. What is it exactly you don't understand? Why not? Because they characterise the system? For instance, how can you calculate how much energy is needed to heat one kg of water from 20°C to 30°C if you don't know its heat capacity? Take two systems, one being 1 kg of water at SATP, the other 2 kg of water at SATP. Is the density different?