Most commonly, they grant +1/+1 to all your other merfolk, but many do even more, like giving abilities (such as islandwalk or ward), drawing you cards, or making your spells cheaper
At this point, the game with the note inside has been in circulation (or in one of our fulfillment centers) for over 18 months, so Im comfortable mentioning it here
Compare with Birthing Pod from New Phyrexia (an artifact with 3+Phyrexian Green Mana casting cost, activation: 1+Phyrexian Green Mana, Tap, Sacrifice a creature: Search your library for a creature card with converted mana cost equal to 1 plus the sacrificed creature's converted mana cost, put that card onto the battlefield, then shuffle your library
Fizzle When a card is countered because there are no longer any legal targets
Ricardo Silva Thoughtseize has been a staple in the Modern metagame for a very long time now and is the undisputed best one-mana discard spell ever printed