A pre-filter sump separates solids and keeps them from entering your filter; therefore, allowing your filter to work more efficiently at removing solubles.
Keeping solids and heavy debris from your water prior to your main filter is the a key to optimizing your filters capacity. The more solids that you can eliminate, the more efficient your filter will perform biologically.
Further pre-filter sumps are one solution to clogged pump baskets and overloaded bio-filters, they aid in removing everything from fish waste to pine needles.