The better your answer to that question, the stronger your deck usually is
Gavin, the lead set designer, noticed that my White Auracite design from the Commander set fulfilled our mechanical needs and had a fairly high-profile concept, so we swiped it (and got an extra mana added to its cost somewhere along the way)
As a rule of thumb, you should consider these when they have natural synergies with your commander
Greninja hits the Bench while Magikarp uses Misty to get to four Energy as fast as possible
Copperhorn Scout lets us untap the team whenever it attacks, giving us pseudo-vigilance
Vigilance Whenever a source an opponent controls deals damage to you or a permanent you control, that source's controller loses 2 life unless they pay