In almost every case they are signify a bad primary relationship.
In the remaining cases.... it's a guy waiting until she's ready to have sex or he gets the balls to leave his current situation. Men generally won't waste time with "emotional affairs" unless they lead to sex.
Women, on the other hand, tend to be
stupid naive in this department. On some level they think a straight guy would listen to them ramble on about feelings, problems, boyfriends, shoes, onyx nail polish, etc... just because he's nice.

Women tend to need (or at the very least feel entitled to) way more attention than men.
In general. Women are more likely to feel an emotional affair means the relationship isn't satisfying. For a guy, it's more than likely about sex or the relationship has too much stress/responsibility.