What are they
Static servers are those which you consider essential or most useful for you. So much, that you want them to be protected so they stay always in your ServersList.
Static servers will not be removed, even if they fail to connect more than the amount of times allowed in Preferences.
If you manually remove a static server from your ServerList, you will be asked to confirm your action, but still, when you restart aMule, that server will appear again.
Are there other advantages in having my preferred servers set as static?
ServerLists can become very large and so, it can become very unlikely that aMule auto connects to your preferred servers. Or even, you might want aMule to only connect to some few servers. By setting this servers to static, you can then, in Preferences, set aMule to autoconnect on startup to static servers only.
How can I remove a static server?
The only way to remove a static server is to set it back to non-static and then remove it.