Latin phrases tend to persist in various specialised areas, particularly associated with the professions (law, medicine etc) which at one time conducted much of their business in this language. If you take out a bank loan, the interest will still be charged at e.g five per cent per annum. Doctors would have told you to take one tablet per diem. In modern times these have become half-assimilated into English, so people might now say "per year" or "per day".
