As found on the NRCS home page for wetlands (see link below), 3 ecosystem services provided by wetlands are as follows:
1. Stores/regulates floodwaters/flooding- Wetlands not only allow water to spread out over a broad expanse of land (as compared to a narrow river channel) but they also contain water dwelling plants which physically slow water down as it moves through the system.
2. Filtering pollutants- wetland soils (peats, bogs, anaerobic soils) contain bacteria and microbes capable of "capturing" many bases (including pollutant & nitrate molecules) from the water solution and using them in anaerobic respiration, therefore removing them from the water supply and sinking them into the soil.
3. Recreation- Wetland environments make for excellent recreation areas for boaters, fishermen, bird watchers, and other wildlife viewing opportunities. These draws increase tourism in the area and therefore hold economic value for the surrounding community.
http://www.nrcs.usda.gov/wps/portal/nrcs/main/national/water/wetlands/
In addition, the Ramsar Convention on Wetlands, an organization responsible for one of the classification systems used in the delineation of wetlands (see link below), offers two more ecosystem services provided by this land type, among many others. These are:
4. Groundwater replenishment- Wetlands can store water in their soils and slowly release it into the ground to refill groundwater aquifers and springs when rainfall is absent and unable to do so. Without wetlands, rainfall would simply move through river systems into the ocean during high precip. times when groundwater containments are full. But in times of low precip., and in the absence of wetlands, there would be no source from which groundwater supplies could be replenished, and those dependent on groundwater for living would be out of luck, because all water would have washed right into the ocean during the most recent rainfall event, instead of being stored in wetlands for later use.
5. Carbon sequestration- Wetlands have very high rates of nutrient cycling and decomposition. As plants grow in wetlands they sequester carbon from the atmosphere during photosynthesis. Due to a continuous supply of water and adaptations allowing them to continue to photosynthesize even during inundation periods, wetland plants can photosynthesize like mad, all the time. There is also a high rate of turnover in the system (death) which offers up lots of detritus to the massive populations of decomposers dwelling in this ecosystem. In this fashion, carbon is locked up in plants and then quickly locked into the soil by the highly active bacteria and other small detritivores.
http://www.ramsar.org/cda/en/ramsar-pubs-info-ecosystem-services/main/ramsar/1-30-103%5E24258_4000_0__
All of these services provide some kind of economic benefit to mankind, either by bringing money into the area (recreation), or by saving money from leaving the area (every other service listed). Without wetlands, a community must invest funding and infrastructure in flood protection and prevention. We would have to invest much more in water filtration plants, restoration efforts, and species population management downstream in order to take pollution inputs into account, or out of the system. Recreation generates funding through tourism dollars, something all too familiar to us here in C.O. Water itself is a necessity of life, and, like tourism, here in C.O. we are dependent on groundwater for our drinking water and irrigation needs. When low snow/rainfall years occur (like this one) our groundwater stores could be slowly replenished over the summer by wetland storage if snowmelt wasn't sufficient. Without wetlands, it could (will) cost the community of Bend and all those dependent on the groundwater in the area (the Deschutes River, for example) lots of money to have to either increase city water rates or restrict farmer access to water, thus increasing food prices next year. Carbon sequestration...climate change, CO2 levels, polar bears, sell your beach house NOW, blablabla, we all know why that's a good thing.