Not sure why you're getting downvoted. I stuck with 10.6 until Linux. Took a look at anything past Lion... "nope!". I despised 10.8 enough to actually downgrade my 2012 MBP to 10.6.
But as for "why update" - up until 10.6, there were performance improvements as well as useful features (subjective -I know- fine).
As for "why update nowadays" - well, 10.6 isn't really supported nowadays. (cough Java 7 cough)