Since I am predominantly a player collector (and someone who grew up in the hobby during the 1990s), I enjoy chasing down the multiple inserts and rainbows of my players. I don't think I'd have much fun in this hobby if I there was only one card of a player I collected released each year.
Here is a trio of 2008 Upper Deck Luke Hochevar cards from my personal collection which came from different sets but appear to employ the same picture.
The top card is from 2008 UD Baseball First Edition, the second is from Series 1, and the bottom card is a Sweet Spot card (and is numbered 002/199).
At first glance, it appears they are all the same photo, but if you look closely at the guy in the navy blue shirt standing in the back ground is walking out of the frame of the picture in the bottom card.
Also, I like how the card from the Sweet Spot release has the stitches of a baseball imprinted on the left-hand side of the card.
These are small differences and changes, but it's the little things that make player collections interesting and fun.