They can enhance talent that was already there, yes. For instance a muscular person might have a crest that gives them the strength to lift a horse or allow them to continue fighting when their body is about to give up.

[There... is something to Rhea's voice. Something that Anna should recognize as disgust.] Crests are based on crest stones. Humans only get them through consuming or being injected with the blood of my kind. Most human lineages who have them stole the blood. Though, some were given up willingly. You, for example, would have likely developed a Crest since you had some of my blood.