To understand why the inspection is so very important, we must first take a look at one example of what is to be inspected. Certain particular areas of a project, or building, that are prone to become the most weak over time is what needs to be the most closely inspected during the construction process. For example, one such area is the joints between timber such as in light wood and heavy wood construction. Also of importance in regards to joints are light gauge steel and heavy steel construction which can sometimes include a lot of on-site welding and many joints are created as a result. The determining factor in these construction areas that can possibly result in great weakness over time is the human element. One must consider that nearly all of the members in a particular light wood framed project, or light gauge steel project, have been pre-engineered and maybe even pre-fabricated to a certain extent. The potential for error occurs when the human is involved. When laborers on the job site are assembling and constructing the pre-made members, there is always a potential for error. The joints, which represent an important structural component in the structure, are also the parts which have, intrinsically, the greatest potential for error and weakness. Keep in mind, this is only one tiny little analysis of one part of a much greater whole, but I think it goes to convey at least in a small way, why construction job-site inspections become so important.
An external party must check and examine the construction of details and joints to make sure the standards of the engineer and of the code are kept.