In recent years, smartwatches' integration with other devices has intensified, paving the way for cohesive multi-device applications. Yet, the depth of their advantages and challenges in this context remains under-explored. Our research delves into the smartwatches' role in multi-mobile device applications. Through a blend of literature review and market analysis, we discerned four potential relationships between smartphones and smartwatches. We further investigated their utility across three distinct scenarios. Interviews substantiated our framework's value, offering insights into optimizing smartwatch use in multi-device contexts.