For example, Quicksilver Fountain says
At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land he or she controls of his or her choice. That land is an Island for as long as it has a flood counter on it.
At the beginning of each end step, if all lands on the battlefield are Islands, remove all flood counters from them.
I imagine this card is useful specifically because it denies the opponent the appropriate color of mana, if they are not already playing a blue deck. If I make a Swamp into an Island, then it produces U instead of B.