They have either no need for lots of mana or they have other ways to cast their spells and generate more mana
A creature with indestructible can't be destroyed by damage.) When this creature enters, another target permanent gains indestructible for as long as you control this creature
Not only that but while in play, Chandras Incinerator effectively doubles your damage spells by copying any non-combat damage done to an opponent and allowing you to then use that to strike down creatures or planeswalkers they control
Mana sources that can become fresh cards later in the game are really good, especially in a format like Commander with such long games
Tenacious Underdog is a staple favorite at the Ayara, First of Locthwain castle, along with Endrek Sahr, Master Breeder and Army of the Damned
1/1 Cat creature with lifelink for Regal Caracal