Why Use UML to Model Components

Software components should be

reused again and again

component assembly and adaptation should be largely automated.
The former requires that components be of a high quality, precisely documented and rigorously developed and tested. The latter can only happen if component descriptions are sufficiently detailed, precise and unambiguous.

To gain the advantages of CBSE, which are:

Complexity Management

Enscapsulation

Separation of Concerns

Reuse