"They are coming in fifteen minutes."
Translation:De kommer om en kvart.
Let's replace the fifteen minutes with sixty: "They are coming in sixty minutes." But nobody would ever say that, because there's a word for sixty minutes in English: "They are coming in an hour." In this case, you do need "an" hour, because now you have a single unit rather than sixty.
Swedish happens to have a word for fifteen minutes - kvart - as well as sixty, and the same logic applies to that word.