Every scientific claim, such as engineering, should be supported by a mathematical model to verify its applicability. Such is the case with structural calculations, which sometimes are provided yet taken for granted in building projects to push through with the construction. But if this is properly done, various realizations in the cost and integrity will come to light, providing more confidence that the undertakings will go smoothly.
Aside from satisfying building permit requirements, structural design calculations can also let you see how the various components were designed and why their sizes were concluded to be as such. One might not easily appreciate it, but considering that it affects around 40% of the total project cost, one should be left with no choice but to question whatsoever is indicated there.