Some voices just have that easygoing charm that instantly makes you want to be friends with the speaker. A cool voice is all about that suave, unflappable quality that lets you know who's the most stylish in the room.
Some people are born cool, and some develop that ineffable quality over time. Regardless, a cool voice lets you know that this person knows what's up, with a natural elegance that just seems to emanate from them in a way that's instantly infectious. You can't touch cool, but you know it when it's there, and these voices have that "it" quality that gives them extra points in likeability, persuasiveness, and charisma. More doors open for cool people and while that may not sound fair, it's part of the package.
What part of your project would benefit from an immediate injection of coolness? Maybe you need to voice a 1950s greaser who needs to remind audiences of Marlon Brando; perhaps you need to use the "rule of cool" to tell an audience that they'll be just as cool if they use it; maybe your project calls for one of those villains that you love despite yourself, because they're so, so cool! Your creativity is the only limit when it comes to how to use a cool voice.
Laid-back, suave, and never in too much of a hurry — that's the epitome of cool. A person with a cool voice always seems to be in complete control of themselves and their surroundings, and it shows in their delivery. Measured, stylish, and definitely charismatic.