Pre-emption prevents damage from happening. This is done either by converting reactive metabolites to less harmful ones, or by speeding up an insufficiently fast chemical reaction. The reactive metabolite can be either a side product, or a normal, but highly reactive intermediate.
For example, a side activity of Rubisco yields small amounts of xylulose-1,5-bisphosphate, which can inhibit Rubisco activity. The CbbY enzyme dephosphorylates xylulose-1,5-bisphosphate to the natural metabolite xylulose-5-phosphate, thereby preventing inhibition of Rubisco.