If, for instance, all “vampires” in your campaign had classic Dracula-esque powers & vulnerabilities, an EC was a great way to standardize them. Ditto “Elves”, “Werewolves”, “Proxima Centaurians”, “Members of the Donnenberg Family”, and so forth.
Part of the reason is that those powers, talents, attribute or talent bonuses would be “always on” and typical of all members of a group, as opposed to being consciously and voluntarily selected for at the time of use. An EC essentially lets you reset the baseline assumptions about _________ away from the standard adult human (10s in all primary attributes, no particular advantages with skills, no talents, no powers, no vulnerabilities or disadvantages).