Schedule 40 and schedule 80 pipe are made from the same material. The difference in structure lies in the thickness of the pipe wall; the higher the schedule, the thicker the wall of the conduit.
The thicker the wall of the conduit, the stronger it is to both outside forces and internal pressure. Any time there is a necessity for a stronger pipe than the standard strength schedule 40, inspectors may require schedule 80 pipe to be used.
More material is used in schedule 80 pipe, making it more costly to produce and more expensive to the end user. In applications where extra strength isn't a large concern, schedule 40 pipe is a less expensive alternative.
The outside diameter remains the same regardless of schedule. Schedule 40 and schedule 80 pipe use the same fittings and same points of entry, making them interchangeable in jobs that don't require a heavier walled pipe.
Schedule 40 pipe weighs less per foot than schedule 80 pipe, which makes it easier to install, especially in long runs.
If a higher schedule pipe is specified in plans or by inspectors, contractors should never substitute the lower schedule for any reason. Using any construction material that won't stand up to the pressure put upon it by regular use or the environment can have very serious consequences.