The most simple solution to pollution is dilution.
Sumps provide this one basic rule in some cases by greatly increasing the water volume of your system.
Think of your 300gal with a 180/125 sitting under it, you nearly doubled your water volume. Not to mention the vastly more space you have for both mech and bio filtration. Can also be used as a retreat for injured fish, hiding place for unsightly heaters etc. If I didn't live in an apt where weight is an issue I would goto sumps myself. We haven't even touched on the ease of cleaning vs cannisters.