Ah the joy of a newly painted room, or remodeled kitchen. There is such appreciation for the excellent work that was performed. No one wants the contant reminder of the damage that could be left behind.
1) Today's surface protection films (plastic sheeting/ polyethylene films) are inexpensive insurance against damaging customer's surfaces within work areas, reducing the amount of additional work and time spent repairing damaged surfaces.
2) Clean-upon job sites is a small fraction of the time when proper surface protection is used, especially when coupled with containment of the work area to keep the mess and dust to a minimal area. Tacky door mats can also be used just outside of the contained area, so any debris from the work area is not tracked into clean areas.
3) Probably the number one reason to use surface protection is to keep the original surfaces intact. A damaged surface can cost more to fix than the work that is being performed.
4) Contractors save time and money on the job sites by using these measures. Workers tend to be more careful, realizing that someone has taken these steps to make sure the job site is kept in perfect condition, which also tends to make subs do a better job within these job sites. The quality of the workmanship actually increases in many cases, because the psychology changes within the work area.
It just makes sense to take care of the surfaces that are not being worked on.
Attribution: Global Plastic Sheeting