The Comet of Power is the symbol of the Celestial Order, which is made up of wizards who study the Lore of the Heavens that comes from the Wind of Azyr. Celestial Wizards, also known as astromancers, spend most of their time gazing into the night skies, plotting the movements of astral bodies. They are experts in divining the future and the most powerful of the order can even scry portents by peering into crystal balls. Celestial Wizards are equally feared and respected for the awesome powers that lie at their disposal; their spells are able to call lightning from the skies and summon meteorites to smite their enemies. The sixteen towers of the Celestial College are the tallest buildings in Altdorf. A shimmering dome of enchanted glass encloses the peak of each spire. The information gathered from these towers is fed into a vast, magical astrolabe at the heart of the college that spins on the head of a mystical silver needle. From this, the masters of the Celestial college read the myriad signs and portents in their bid to understand the Empire's potential futures.
Wind Blast is probably the best spell in the game for clearing out low armor infantry, so if you are fighting a faction with unarmored infantry you can't go wrong with the Celestial Wizard. They also are useful for damaging archer or missile lines as they tend to remain stationary while they fire. They will be good against Greenskins, Tomb Kings, Vampire Counts, Vampire Coast, Bretonnia, and probably High Elves. They will be less useful against Chaos, Dwarfs, and Lizardmen. Because of the Lore attribute, flying units are weaker when these wizards on the battlefield. While it is easy to initially think of big scary flying units like Royal Pegasus Knights, or Star Dragons, this can also be useful for the lighter units such as Fell Bats or Carrion. If you expect flyers, you can't go wrong with a Celestial Wizard around.