Let's get controversial.
Formal business letters are not more professional. They're just odd. They're not better service; they're aloof and cold and say, 'This is our routine letter because we couldn't be bothered writing one especially for you'. Any manager who insists on formal language for business letters and reports is wrong.
Ask professional writers to describe effective writing and they'll give you more answers than you need.
That might suggest that they don't agree on what makes writing effective. But they do agree. They just use different language to describe what they do and like to read.