Component and services enable reuse and encapsulation of responsibilities and implementation details.
They come into play when a monolithic application and a monolithic code-base is not enough anymore.
These are the 6 forgot OO design principles about packaging cohesion & coupling that suggest criteria to group classes and responsibilities in components and services :
REP The Release Reuse Equivalency Principle The granule of reuse is the granule of release.
CCP ...