YES.  I had to take this question to the felt, and the below answers represent majority of why players think opponents show:

1.   Fugu123 (Canadian) To make your opponent think what you think they will think  when they see your   cards;

2.   Kobi894 (Russian) To brag; or

3.  (anonymous) a misclick.

TIP:  NEVER SHOW  (badbones141)  Showing is a sign of weakness. Showing for free means they dont have to pay to see . Any reason anyone can come up with for showing.......there is a greater reason not to show.

So is showing cards important?   Oh yeah  - NEVER SHOW,   EVER.