I assume you mean historically, and I will address that first. If you mean tactically I will answer that at the end.

Originally, the piece we call a queen was called a "minister" or "vizir" in various languages. It could move one square diagonally, making it a weak piece and making pawn promotion less important. Incidentally, this made check mating your opponent much more difficult. We think this piece was renamed "queen" between 1475 and 1495 CE and that the piece's scope was redefined at the same time. This likely occurred in Spain, but could have occurred in Italy, France, etc.

Some ascribe this change to Queen Isabella of Spain, particularly after her role in the Siege of Málaga (1487), but Marilyn Yalom cites earlier depictions of the Virgin Mary as the chess-queen of God (112-114), and this could have also inspired the change. What we do know is the oldest surviving book detailing the queen's new movement is Lucena's 1497 book Repetition of Love and the Art of Playing Chess.

This change was not adopted quickly and universally, though some have argued the Reconquista and the advent of the printing press helped spread these changes. And this still was not the modern version of chess we play today; important changes like rules on stalemate and castling would come later.

Sources:

Yalom, Marilyn (2004). Birth of the Chess Queen: a History (Illustrated ed.). HarperCollins. ISBN 0-06-009064-2.

Murray, H. J. R. (1913). A History of Chess. Oxford University Press. ISBN 0-19-827403-3.

Now regarding the tactical importance of the queen, it comes from its mobility. A queen in the center of the board can directly influence 27/63 other squares. To put that in perspective, the next most mobile piece, the rook, can influence 14/63. This makes the queen nearly twice as powerful, and is reflected in their assumed "point" values: queen=9, rook=5.