Some abilities, such as convoke, allow you to tap untapped creatures to pay costs
Today, Tannuk is the last remaining member of Sami's crew after the rest split for a more lucrative ship
This can +2 to draw a card and scry two which is nice card flow, and then -3 to exile the top card of your library and then make a number of 1/1 flyers equal to the mana value, which is also very strong in Type Four with its elevated casting costs and low land count
Captain Mar-Vell, Space-Born Final-Word Phantom Heliod, the Warped Eclipse High Fae Trickster Leyline of Anticipation Quicksilver, Speedster Tidal Barracuda Vedalken Orrery
For example, Overkill's mana value is 3, because it costs two generic mana and one black mana