Oh I know but it kind of evolved to keepers shouting it the second the corner was in the air which always seemed kind of obvious
Keepers making a call as the kicker's boot hits the ball are afraid of aerial duels.
My high school coach developed a scheme that I doubt would have worked at higher levels, but it was nice at the time. If I clobbered somebody on a corner or free kick early on, we'd change up the call. The standard "Keeper" became the "away." If a ball was borderline or in the area I might
come get it, I'd make that call and hope it gave somebody pause. It was probably only a fraction of opponents that would be intimated by the memory of the earlier collision, but it likely bought my defenders a few unchallenged clearances over the course of the season. Everybody knew I hated the term "goalie" for my position, so naturally the call for me to take the ball became "Goalie!" Assholes.
And I'm sure by telling the forum this story, I'll see "goalie" all over the fucking place now.