Tierranking is usefaul feature because:
Helps organize your own thoughts: Tags make you articulate why you rate a card highly or poorly, rather than relying only on an overall gut feeling.
Makes evaluations easier to understand: Other players can see what aspect of the card you value—e.g. synergy, flexibility, removal, consistency, or power level.
Makes comparisons more meaningful: Two cards with the same tier rating can be strong for very different reasons, which tags make visible.
example attached