The critical purpose is to reveal the grade of the wood (number of
knots, etc). Load-bearing applications like studs and joists need a
certain degree of freedom from defects to reliably stand up to their
rated loads. This can't be seen in rough-sawn condition.
In olden days, yes, rough lumber was indeed used for construction, at
least into the 1930s. This caused a lot of problems. I once lived in
an older home that had a buckled wall. We discovered the cause was a
rough-sawn joist that had cracked from a knot most of the way through,
but invisible from the rough surface.
It is also not necessarily "cheaper" to use rough-sawn wood, because
lumber is cheapest when it is properly graded and sorted, and then
priced based on that grade. Using clear lumber on studs instead of
finish work is not cheap, nor is using flawed pieces in a load-bearing
structure. Hamburger is cheap because it isn't made from filet mignon.
Every tree has a variety of cuts and quality in it, the grading and
sorting is a vital part of what makes lumber a commercial product.
By the way, dimensional lumber is surfaced by a planer to smoothness,
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.