To further specify your question, correct me if I'm wrong:
Assuming you let a machine sit idle after using it, is there an additional security benefit to logging off, in terms of network attacks?
The theoretical answer is "yes". Software that is running when you are logged in can pose a security threat by receiving network traffic that exploits vulnerabilities in said software. When that software is running in your user space, that vulnerability goes away when you log off.
That's for outgoing connections, but incoming connections can pose the same threat. That is, if your router and firewall (if any, and enabled) allow them to even reach your machine and get sent to the listening application.
There are countless of other programs that could be running after you've used the machine, each of which could have networking functionality (either listening for traffic or actively communicating with other servers) that could contain vulnerabilities.
Even if you kill all visible applications, there is always something extra running that could theoretically get attacked while you are logged on.
Logging off stops these programs.
Logging off also sets the baseline: when your machine displays the login screen, countless of background programs (services) are running, a lot of which can still invoke network traffic and get attacked.
And if your machine is idle and nobody will be using it for hours anyway, why not turn it off completely?