What is the "apache restart interval" referred to in Server > Server preferences?

I had been under the impression that Plesk is supposed to restart apache periodically, according to the number of seconds entered in this field, but that doesn't seem to be the case, at least under 7.5.2. I've looked at this on four different installs of Plesk, all with the interval set to 900, and on all of them httpd has been running for hours, days, or weeks without a restart (both according to 'ps' and to the mod_status-generated server-status page).

So what's it actually for? Or is this feature just broken?

Thats the interval to restart the webserver when someone makes a web related change. Previously PSA would just restart the webserver whenever any change was made, and on really high volume websites, it would cause a DoS situation.

But then shouldn't it be the case that if I add a domain, apache should restart within that amount of time?

If I add a domain, and wait an hour, apache still won't have restarted. I always have to restart apache by hand to make changes take effect.

correct, that is the way it is supposed to work. If its not then Id say something is wrong with PSA