This question is a little vague...
I guess that the most obvious reason that a watershed is important is because the conditions in the watershed to a lot to determine the health and quality of the body of water where all the runoff from the watershed ends up.
For example, the Mississippi River watershed (there's a big one) has a lot of farming. Because of this, it has a lot of agricultural chemicals in its runoff. These chemicals have caused a huge dead spot in the Gulf of Mexico.
I hope that's the sort of thing you're asking. It's not very clear to me...
A basin like landform defined by highpoints and ridgelines that descend into lower elevations and stream valleys is known as watershed. A watershed carries water come from the land after rain falls and snow melts.Drop by drop water is channeled ito soils,groundwaters and streams making its way to larger rivers and at last the sea.
Watershed comes in all shapes and sizes.Farms,homes,big cities can make up watershed.
Watersheds are important because they help sustain life in a particular ecosystem. They are an awesome indicator of the health and cleaness of an ecosystem. The rain falls on the land, runs across the land picking up any pollution and trash, and it eventually all can be found in the watershed. If the watershed is low, then drought is occuring and the land it is found is dry also, effecting all the plants and animals around it.