Flaked wheat is still raw wheat; it has been pre-gelatanized which makes it suitable for a single infusion mash or for a minimash. Raw wheat is very difficult to crush, and requires a protein rest or cereal mash to gelatanize the starches.
While many argue raw winter wheat will make the best Wit, flaked wheat is also quite acceptable. Wheat malt is OK in a Wit grist as long as you use at least 25% raw wheat. Using all wheat malt in a Wit won't give you the tartness you should get in a Witbier.