Why is a watershed important?definition- a watershed is the name for an area of land in which water drains not a specific river.

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...