Here's an example. Most toxicologists looking at the effects of heavy metals on biological systems (e.g., phytoplankton or fish gill) will assume that the toxic form of most metals is the free or aquo species (Me²⁺). Hydrolysis products (i.e., MeOH⁺, Me(OH)₂, Me(OH)₃^-, etc.) or complexes with other ligands (e.g., SO₄^2-, PO₄^3-, or CO₃^2-) are generally considered to be less harmful. If you measure the pH and the total concentration of the different chemical components in your system, you could use chemical equilibrium modeling to calculate the amount of free metal ion that is present in your system.

This is a vast improvement over just reporting the total concentration of a metal in relation to biological response. By accounting for pH, all side reactions and the competition between metals for ligands, you are able to arrive at a much more insightful description of the chemical environment -- and a much greater ability to form hypotheses about how metals interact with organisms.