To address "why care if someone else believes in something imaginary"... really it's embarrassing that it even needs to be explained.
When one man is motivated to destroy another because the imaginary things they believe in differ, there is a reason to care.
When one man condemns another to a sub-human status because they don't live according to the things proscribed by their imaginary thing, there is a reason to care.
When we all have to share the same air with these people, there is a problem. Aggressive atheism seeks to change other people because those people can't sit still and let everyone else live their lives. I have nothing wrong with spirituality (I think it's still sort of pathetic but harmless anyway), it's when people damn the lives of others simply because they don't share the same mental deficiency ("faith") that I feel like there's an evil in the world that needs to be destroyed.