Brake line flaring has to be created on some kinds of brake tubes. There are many kinds of brake lines. Like basic rubber tubing; braided tubing that is made of layers of Teflon, covered with braided steel tubing; and purely metal tubes. Basic rubber tubing and steel braided brake lines do not require flaring, but fully metal lines do. Most modern brake lines are made of stainless steel coated with tin or zinc or made of copper-nickel alloy. Usually, stores sell commercially made brake lines that are pre-flared for use. But it does become necessary to cut the lines to adjust to individual cars. As a result, you might have to create the flare yourself or get a garage to do the flaring.
For more info, see How to Double Flare a Brake Line