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
Extra removal and a free bolt during each combat wont mean much when there's a 10/10 Orc Army with deathtouch, menace, and ward 2
I cant confirm or deny these numbers, but in my opinion, these numbers when compared to each other make some sense
Running pro-red creatures like Auriok Champion, or spells and equipment that grant protection its paying off for me when it comes to weathering the storm
(Zero is even.) Each other colorless creature you control has bloodthirst 2
While some land fetchers only grab basics, such as Solemn Simulacrum and Cultivate, there are enough Wood Elves and Polluted Delta and similar cards out there that will grab three colors in one land fetching ability