Why does a conserved domain not establish that two proteins share the same biological function?

Why

Sequence search often finds a familiar catalytic or binding domain in proteins that act in different pathways. Domain matches can guide hypotheses about a fold or molecular activity but usually cannot determine cellular role on their own. For example, two kinases may retain an ATP-binding domain while differing in substrate selectivity, localization and regulatory partners.

Examine full-length architecture, signature catalytic residues, paralogy, species context, subcellular targeting and gene neighborhood where relevant. Seek orthogonal evidence such as phenotype rescue, substrate assays and interaction experiments before assigning a detailed annotation. Annotating an uncharacterized sequence from its closest named database hit risks propagating older annotation errors.

State the confidence and granularity of each proposed function separately.