While a simple "Hi," "Hello," or even "Hey" is appropriate in casual correspondence, a more formal salutation is appropriate when you are emailing about a business_related matter, such as a cover letter, letter of recommendation, or inquiry letter.
Consider the daily life of whomever you’re sending a letter to _how many other things might they have on their mind when they read your letter. Maybe they have a meeting in five minutes. Maybe yours is one of 200 letters they have to wade through. Maybe it’s the end of the day and they’re worn out. Giving them a concrete action to take makes it all the more likely that it will “stick” _ they’ll add it to their todo list or their calendar, they’ll pick up the phone, they’ll check out your website, or they’ll respond.