Why territorial waters still need blue-water reach

If territorial waters are the waters a state must defend, why does it need a blue-ocean fleet?

The answer is that threats can attack from well beyond territorial waters, so defense cannot be limited to the legal boundary line.

Territorial waters are a legal boundary, while weapons and launch platforms can operate far beyond it. A state may already be within engagement range before a threat crosses that line. Defence planning therefore has to account for military reach beyond the waters legally defined as national territory.