Scrap doesn’t just happen
In reality, if you need 1,000 of whatever product, the manufacturing process is rarely going to yield exactly 1,000 of it, even if you feed into the first operation the exact quantities of raw materials system calculated as gross requirements. The process may produce 980 or 1,020, but is hardly ever going to be exactly 1,000. If you didn’t take scrap into account, or you took incorrect scrap into account, your actual output from the process might be much more unpredictable, with huge variances. Variances are always a problem, only they don’t have to be that big a problem in all scenarios.