There are two basic types of fear: innate and learned. Innate fear is more or less fears that you're born with. An example would be a fear of heights since the act of being up on a high place causes the brain to realize that you may fall and suffer damage. It's a defense mechanism to keep the person from hurting themself. So this is one example of it being a positive benefit.
On the other hand, it's destructive because it could prevent a person from enjoying life such as going on a hike or going into a tall building. Another example of a destructive fear is any kind of learned fear that hinders a person's lifestyle and enjoyment. A learned fear is exactly that: something that your brain learns. It's based on classical and/or operant conditioning.
A famous example of this is the psychology experiments of Little Albert in 1920. These were a series of experiments aimed to show that fears aren't always innate. To show this, they somehow tied the boy's fear of white rats to a fear of white cotton. So basically any time the boy saw balls of fluffy cotton, he would get terrified. This experiment is easy to see why it's unethical. Also, it's an example of a destructive fear to get in the way of life.
