I'm going to assume you've been close to a field during warm-up pitches. I'm going to assume you've been near a 90 MPH pitch.
Velocity stands out. The sound of the ball cutting through the air is audible. The "POP" of the catchers mitt is so distinct. If you've seen a pitcher throw hard, you've heard a pitcher throw hard. It's music to a baseball fan's ears.
The industry has become nearly obsessed with with velocity and has invested a lot of money into finding more of it. The use of high-speed cameras, weighted balls, bands, and other tools is getting close to becoming common place.