There are a few reasons why I post...

-To offer support
-To offer my input/advice when it is requested
-To welcome new members
-To receive support and/or input & advice when I ask
-To learn new things...for example, I'll sometimes create threads in the Pets section to learn more about the behaviour of my puppy, Zeppelin. Always interesting to learn something new!
-Occasionally, to post something I find interesting (occasionally post a song I really like in the music section, for example)