Chess: The Most Popular First Move for White and Why?

Pawn to King 4. (also written as P-K4 or in algebraic notation: e2-e4 or simply e4)

This is the most common single opening move in all of chess.

The reason for this is that it offers you the most coverage.

As you can see below, the bishop is protecting 5 squares and the queen covers an additional 3. The pawn also attacks 2 squares, one of which is in the center. The rest of your pawns are covering the entire third row That's 18 squares. (almost 1/3 of the board).

If you're black, the counter move is move is to move your king pawn up to block the white pawn. The same reasons apply.