Typically in fantasy manga (or manhwa/manhua) the important characters' attires serve fanservice needs. In shounen/seinen, they need to be either sexy or cute (for female characters), depending on the series theme and publication requirements. Also depending on the title, it might be necessary to have the male MC have cool clothing, but often it's not of particular importance. In shoujo/josei, clothes need to be fabulous, so that the female readers can admire them, which is also fanservice. Functionality is of secondary importance. Of course there are exceptions, but even that exception has its own expection, which usually means a helmet doesn't exist for the characters' combat apparel, even though martially and historically helmet is one of the most important pieces of protective gear. But you can't hide the faces and hairdos of important characters!