"Shall we play a game?"

"How about a nice game of Chess?"

D5 is a Chess move reference!

"To be a professional chess player is to spend your life living out of suitcases and fighting for control of the D5 square."

The reason D5 is more "important" is simply because it is a popular square of contestation in the Sicilian, arguably the most popular opening in the world.

It is also an important square in many other opening lines such as the Queen's gambit, as pointed out above.

Whether or not "important" is the correct word is debatable. Certainly in these Sicilian lines, the D5 square is more important than any other central square, but of course D5's importance is dependent on the board position.

I would not agree that all center squares are equally important because, for example, D4 is rarely a square that people fight over.