F4 for white and F5 for black are often very important moves in the middlegame.
Very often - in fact most of the time - one or both Kings are castled short on that side of the board.
And then the f4 pawn is the center and spearhead of the Kside attack.
With an e4 pawn (or advanced to e5 pawn) and a g4 pawn on either side of it.
White 'comes down' on the black position - and he supports his pawn attack heavily with his Queen and knight or knights and bishop or bishops - with or without his rooks getting into it too - or instead.
Without his g-pawn supporting his push to f5 - then the white pawn at f5 could often just be captured and then its black with a big pawn or piece at f5.
But with his g4 pawn backing up his f5 pawn - then that gpawn can recapture at f5 and becomes the new f5 pawn that's like an arrow sticking into the black position ...
unless it can be immediately punished then white can often 'mop up' even with black castlled long.
To understand 1) f4 better - it would help to understand why 1) d4 and 1) e4 are both better first moves. By understanding how they pertain better to the opening arrangement of the 32 pieces - which is called the Grand position.
I cover secondary (less common) moves against 1 f4 with lines 11 through 17 with my blog repertoire.
Lurching Chess - Chess.com
Here is a Bird Opening game I played this morning.
Good Chess! Keith