High Sierra Topix

I have noticed that if I do not "log out" this is automatically done when I turn off the computer. Is there any reason to officially "log out"? Does it protect me from computer viruses?

There is no real reason to log out other than the refreshing of session cookies. The session timeout length of the forum itself is really long...days or weeks I think. What is logging you out is likely a setting in your browser, not the site.