You are correct in your assumption that corrugations give pipes strength and hence such pipes can have thinner walls that smooth walled pipes made of the same material when used for the same application.
Pipelines are made to transport fluids or slurries. When it comes to the flow of fluids or slurries in a pipe, one of the import considerations is the smoothness, or roughness, of the internal wall of the pipes. Smooth walled pipes, or as you called them “plain pipes” have a small surface roughness on their internal walls. The corrugations of corrugated pipes are a very large roughness. This will increase the resistance to flow within the pipes and increase the turbulence of flow in the pipe. Increased turbulent flow, particularly of slurries, can lead to increased wear of pipes. The smoother the flow in pipes the better.
Sometimes, the flow of sewage in pipes is not a continuous steady stream. As the spasmodic flow subsides the sewage fluid and solids will collect in the valleys of the corrugations. If the corrugated pipes are made of steel, this will lead to a higher probability of the pipes corroding more quickly in these locations. The other result will be the presence of decomposing sewage and fluid trapped in the corrugations which will lead to the creation of a habitat for harmful bacteria and the production of noxious gases. The result of which would be an increased risk to public health. To prevent this, such pipes will need periodic flushing.
With smooth walled pipes there will be less accumulation of sewage based items in the pipe as it will tend to form a narrower line along the center of the base of the pipe. Corrugations run transverse to the pipe, allowing for more material to collect in bottom of the pipe.
The wall thickness of any pipe will be determined by the strength properties of the material from which the pipe is made and the pressures that the pipe must withstand; both internal and external pressure. The deeper a pipe is buried the greater the soil/ground pressure that it must withstand. HDPE is a weak material, a wall thickness of 40 mm for a 500 mm diameter pipe means it will have to withstand high pressures but it also helps the pipe keep its shape.