In Reuben Fine's 'The Ideas Behind Chess Openings' page 7 he wrote:

'OPENINGS WITH 1 P-K4, P-K4

Both White's and Black's initial moves are perfectly natural and
normal: both assist development and affect vital central squares.
As long as Black can retain symmetry, White can lay no claim to an
advantage. Consequently the task is to compel the defender to give up
his strong center positions, in other words to abandon his P at K4.
White can achieve this aim only by playing P-Q4.'

Why?

'If Black then replies with PXP he will be left with a pawn at Q3
(eventually)...'

Why?