Simply put, some items are more valuable than others. Therefore, these more-valuable items should produce more/be more likely to produce scrap in a Recycler.
For example, if you dumped in Tin Dust it would produce more scrap than, say, cobblestone, and the extra scrap can be used either in scrap boxes or a massfab to get other useful items.
In short, it would allow transmutation of what you have a lot of (tin, copper) into other stuff you have little of (iron) at a large trade disadvantage.
Alternatively, the valuable items could be fed directly into a massfab amplifier slot with a custom amplifier amount, similar to how scrapboxes are worth 9x as much as individual scrap.