[kemayo]

Depends on the specific thing, but generally it's because there might be consequences to doing things poorly that would spill onto your neighbors, or which might cost the local government money to fix after it hurts or kills you. It's the same kind of reason that laws requiring motorcycle helmets or seatbelts exist.

The sewer one is obvious, since it could be a public health issue -- if you screw it up you could wind up contaminating local water sources (or just stinking up the area). Issues with the lumber you use, or with your building plans, could potentially result in your house collapsing in such a way that it might damage your neighbor's property. And getting a new permit if you take too long is probably just a way to force you to check in and make sure that you're not deviating from the earlier plans you filed.