Some players will show to influence their opponents actions later in the game. This is a perfectly acceptable strategy. Most strategy says to always muck just because that's the best play for the average player that shouldn't try to get too tricky. Personally, I have all my sites set to auto muck, but I will occasionally show I was against a player I really like or if I had a really good hand I just want to share with my fellow players. If I have something like quads or a straight flush and it's a relaxed game I'll show just to show off. =)