What is density? Why we have to give more importance for density in EM thoery

What is this density? why we have to give always importance in Electromagnetic theory such as Electric flux density, Electric field intensity, volume density,charge density,line density.....

Why we are always discussing densities in EM theory.

Because excess density affects material properties adversely, it is an important attribute. It is the limiting factor for many properties such as light emitting diodes, heat spreaders and safety of EMI absorption in human flesh.