Transformation (変身 Henshin, the English term Set Up is used synonymously in the franchise), although ubiquitous in magical fantasy productions (see Henshin), is not a specific magical power or ability in the Nanoha universe. In the Mid-Childan magical science, "transformation" refers to the simultaneous set-up of a mage's Device and Barrier Jacket, which can also be activated separately. Therefore it is very common to see characters with set-up Device but without Barrier Jacket, or vice versa.
Even though transformation sequences are produced for most of the main characters in the series, they are only fully displayed once or twice before being replaced by a generic flash of the characters' respective magic colors to conserve anime screen time and manga page space. Since transformation sequences commonly feature female nudity, their main purpose seems to be that of a commercial gimmick and fan service. Some characters' transformation sequences are never depicted but merely indicated with a light flashes, e.g. Reinforce Zwei and Ginga Nakajima in StrikerS. It is implied that the full transformation sequence is a slowed-down of the actual transformation which takes less than a second and appears as a brief flash to an outside observer.