User blog comment:Leek0421/Dragon Slayers & Celestial Magic/@comment-75.172.82.63-20150501152711/@comment-7418318-20150501171926

They explained it in the series itself.

Celestial Spirit Magic is all about opening a door through time and space. Time because of the different flow of time in the Celestial Spirit World and space because it's a different dimension.

Any other magic in Fairy Tail that we know of deals only with space OR with time (or neither of the two). Only Celestial Spirit Magic deals with both time AND space, thus it becomes incredibly useful for opening portals, altering time-flows and such stuff.