Structural Design and Drawings are very important for buildings as heavy reinforced (excess steel) caused sudden failure (without any warnings) in the buildings during Earthquake while lower side provides less strength to the buildings. Both above cases are very dangerous to our Residence/property where we live.
The aim of designing the building is to design buildings so that it fulfils its purpose during its intended lifetime with adequate safety (strength and stability) serviceability (durability and stiffness) and economy keeping in mind the quality parameter of the materials and workmanship as per Indian standard.
Safety implies that the possibility of (partial or total) Failure/collapse of the structure is acceptably low not only under the normal loads but also under probable overloads such as earthquake and extreme wind.
Serviceability implies satisfactory performance of the structure under service loads, without discomfort to the user due to excessive deflection, cracking, vibration etc.
Economy implies satisfactory use of money (lowest cost) without compromising the strength and quality of the building materials. This all are only possible when we use proper structural drawings with exact reinforcement detailed showing placing and bindings of steel and work accordingly.
On the other hand if we need any changes/alteration in future, we can do the same without any hesitation and flip flop. (i.e. what is the steel beneath the column and beams, Can this structure is sufficiently strong to bear this much extra load). Because every thing is known to you and it is in your record. Otherwise we will have to take the help of some technology to find out the quality and strength of building, which is very costly