"What is the advantage of using SMD components in an assembly?"
Common components do the same things as SMD equivalents. A common (discrete) resistor has the same electrical properties as an SMD. However, SMDs are designed to be used in machine assemblies, which can result in much smaller as well as more reliable boards. You can perfectly develop a project with common components and once it works, if you want to industrialize it, have the boards made with SMD components.
