Why is pitch an important property of music?

"Pitch" is the impression we get from the frequency of the sound.

'High pitch' is the sound of a piccolo, a mouse's squeek, or a little girl's voice.

'Low pitch' is the sound of a tuba, a big drum, or distant thunder.

The system of pitch is the whole idea of the 'key' in music.

If the singer hits the 'wrong note', he sounds 'off key' ... that usually means

that he's singing the wrong frequency, and we hear a pitch that doesn't belong.

The pitch is important because of how low or high or how loud would it be. the frequency of the sound for human ear.