The same reason that people do that when they have babies or get a different car I suppose. What's the point of a Facebook account if you're not using it to scream out to the world, "look at me, notice me, pay attention to me!"
Look at the bright side, think of how funny it will be to nursing home attendants who are bathing inked up 80 year old grandmas with tramp stamps.
To show it off? You've probably shown off something you like once or twice, Facebook or no. Why look down on them for getting excited over something different?
They are begging for attention.