Marshes give off saturated soil: "Marshes, also known as wetlands, are frequently inundated (flooded) with water. Plants found in marshes such as the common cattail, lily pads, and duckweed are adapted to the saturated soil conditions. Animals commonly seen in a marsh are otters and muskrats. There are many different types of marshes including coastal, inland, fresh water, salt water, riparian, and more. Marshes get their water from surface water (rain/runoff) and ground water." (Gundersen Envision,p.1)
They have plants and food for living animals there to eat.
There are fish for humans to use as food for.
They Drain common water from floods.
Most have fresh water (Meaning its drinkable)