One of the jobs of a civil engineer is to make sure the structure being built is the same as the one that was planned. What's being built should have no shortcuts taken and no substitutes made without being resubmitted for new approval. If you don't have a civil engineer overseeing the construction, you might not know about changes that the construction crew made, either on purpose, like a shortcut, or by accident, such as misunderstanding something in the plans. That can lead to legal trouble later on.
Those Plans Were Pre-approved
The plans submitted for the project were approved specifically because those plans met local, state and national laws and guidelines and did not create problems for the surrounding area. If the construction crews change something in the structure and veer away from what the plan said would be there or what would be used, the new feature could violate codes or laws.
Last-Minute Changes Could Interfere With Other Parts of the Plans
The plans for the structure, whether it's a building, car park, or another structure, were created so that everything in the plan works. Many times that smooth sailing is literally dependent on everything working as planned; for example, people being able to get out of a car park safely depends on there not being obstructions blocking their view of oncoming traffic. If you put an obstruction there, the car park plans don't work as well. Any changes need to be approved, and if they aren't, it's the civil engineer's job to get those approvals and get the project back on track.
Future Legal Issues May Hinge on Those Plans
The plans for the structure are a record of what the structure contains and how it was built. If something were to occur in the future, such as if certain materials used in construction were found to be toxic, and the plans for the structure showed none of those materials were in the structure, then anyone working in or near the structure would know that they didn't have to worry about those toxic materials. But if the construction crew took a shortcut and used the material that later turned out to be toxic — and they hadn't sent the plans back for approval — then that structure could be more dangerous than people realised. A civil construction engineer ensures that what you see on the plans is what you get in real life.