Teacher Discussion Forums

Someone on the Korean forum asked about the function of "by" in "increased by 10%" and such sentences.

This "by" is obviously used in conjuntion with verbs which measure a change of quantity, but it seems always omissible, and therefore does it have a function? Are there other lazy prepositions of the same type?

These phrases are quite often passive, so is there a passive-linked reason for this by?

I suppose 'see you Saturday' sees an acceptable (or at least common) ommission of a lazy 'on'.