how many land cards in a mtg deck Rin and Seri, Inseparable The Guide to Playing Utility

Besides being able to run Kethis, the Hidden Hand, a number of other Abzan legends use +1/+1 counters to really double-down on Reyhans ability

this allows them to evolve a non-Floodborn character into their new form, for a reduced cost to play the Floodborn card as a bonus

Individually these effects are marginal: but combined they form a card that was so good, many non-red blue base decks started playing red just to get it (plus Flametongue Kavu, to be perfectly fair)

Ability costs can include ink (paid by exerting cards in your inkwell), exerted cards (paid by, well, exerting cards) or something else explained on the card itself