A wetland looks, to the casual eye, like unused land waiting to be drained or built on. In practice it is doing some of the most valuable work in an entire landscape: filtering pollutants out of water as it passes through, storing floodwater during heavy rain, and slowly releasing it during drier periods.
That filtering function alone can rival the cost of a built water treatment system, except a healthy wetland does it continuously and without ongoing engineering, simply through the plants and soil already in place doing what they have done for a very long time, quietly, and largely without anyone noticing the work being done.
Wetlands are also disproportionately rich in biodiversity relative to their size, offering breeding grounds for fish, resting points for migratory birds, and habitat for species that exist almost nowhere else, which is part of why their loss tends to ripple outward far beyond the boundary of the wetland itself.