Water Supply
The biggest reason watersheds are important is that they provide water to the larger bodies of water. The Chesapeake Bay is supplied by a 64,000-square-mile watershed. If this watershed didn't exist, the lake would quickly shrink in size and lose a lot of valuable minerals that keep it healthy. All the cities along the Chesapeake Bay depend on the watersheds to supply the lake which supplies them with water.
Land Drainage
Watersheds do more than just feed lakes and seas with water, they also drain the land of excess water. If the water were not drained regularly, plains could become marshes or bogs. This would have disastrous effects on farms or infrastructure built on the land. Additionally, the current ecosystem could be wiped out if the land is permanently flooded. Animals can suffer infections from bacteria that breed in standing pools of water.
Pollution
A growing reason why watersheds are important is that any pollution in the tributary feeds will end up in the bigger bodies of water. If a factory dumps chemicals into a river, those chemicals will eventually end up in a lake or sea. All the cities that draw their water from those areas will then suffer from the pollution. At best, the cities have to spend more money on water purification; at worst, they may have to deal with illnesses or deaths the pollution causes.
Safeguarding
Watersheds need to be safeguarded to prevent their damage. Construction must always take into consideration the path of tributary streams. Farmers need to know how their fertilizer will affect organisms living in the water. Companies need to dispose of waste responsibly. It takes a combined effort to protect watersheds from damage.