Q. Is it always necessary to use an en dash while it is representing a range of numbers (15–30)?

A. No, but it’s Chicago style. Since en dashes used to be available only to typesetters, typists have long used hyphens instead. Even with word processors, many people don’t know how to produce an en dash. Many editors seek the abolition of en dashes, believing that they serve no purpose that couldn’t be served by other punctuation. (Legal writing is an arguable exception.)