The reef aquarist in particular is always trying to maintain high quality seawater. So is the fish only aquarist, though the quality demand is not quite so vital.
The aquarist test the seawater parameters as a matter of course. Most aquarists find that all is well, or can adjust to remedy an incorrect reading. There are aquarists though who use high quality salt and are particular about the [tag-tec]aquarium maintenance[/tag-tec]. Despite this, they find that problems arise, often in the from of phosphate or nitrate. They then obtain filters to deal with these, creating more expense and more maintenance. There is nothing wrong with getting rid of undesired substances with specific filtration methods.
The question is, where are those undesired substances coming from. As said, if the aquarist is doing everything correctly, including feeding, it is unlikely that it is being generated within the aquarium. So where does it come from.