Knowing your Finger Numbers on each hand is important, because when playing the music you have to know which hand and which finger plays a certain note on the keyboard.
That is why we number our thumbs and our fingers.

Thumb = Finger No. 1
Index Finger = Finger No. 2
Middle Finger = Finger No. 3
Ring Finger = Finger No.4
Little Pinky = Finger No.5